Sensed presence: the feeling that someone is there

A sensed presence (also "felt presence" or "feeling of a presence") is the conviction that another being is close by when nobody can be seen or heard. It is the effect the God Helmet is best known for. Persinger's laboratory said its weak fields could evoke it on demand. The one rigorous independent replication found that the field made no difference, and that personality and setting predicted who felt it.

The same experience turns up far from any helmet: in sleep paralysis, after a bereavement, in Parkinson's disease, on high mountains and polar journeys, and in neurological patients. Two groups outside Persinger's lab have produced it reliably, one with electrodes inside the brain and one with a robot that touches people's backs.

This page covers:

• what people report, and how often;
• the Persinger/Koren laboratory record on presence;
• the independent tests and the suggestion explanation;
• sensed presence without any helmet, and the proposed mechanisms;
• videos worth watching;
• a reading list;
• how to test it honestly with this helmet.

Related documents:

• RESEARCH.md: the full evidence review, including every lab protocol cited here.
• EXPERIMENTS.md: blinded real/sham sessions, checking the blind, and the replication designs.
• SAFETY.md: who should not use the helmet, session rules and the adverse-event record.
• QUESTIONNAIRES.md: the Sensed Presence Questionnaire (SenPQ) and the post-session checklist.
• HISTORY.md: the people and institutions behind the God Helmet.
• SIGNALS.md and PLACEMENT.md: the signals and coil sites the programs use.
• The two other effect articles: Out-of-body experiences and Remote viewing.

Evidence words, as in HISTORY.md:

At a glance

What people report

The experience

A recent clinical review defines felt presence as the basic feeling that someone else is present in the immediate environment without clear sensory evidence. It ranges from benevolent to distressing and from a definite person to something vague (Barnby et al. 2023). Accounts from very different settings share a set of features:

How often it happens

The figures come from different questions and samples, so they cannot be compared directly. They do show that the experience is not rare and does not need a helmet.

The God Helmet laboratory record

The hypothesis

• 1983. Persinger proposed that mystical and religious experiences come from brief electrical "microseizures" in the deep temporal lobes, and that life crises and deathbed conditions are optimal triggers (Persinger 1983; Claim: Stated by an author, vendor or programme, without published supporting data.).
• 1993. His "vectorial hemisphericity" model made the sensed presence the starting point. When the hemispheres briefly fall out of step, the right-hemisphere equivalent of the left hemisphere's linguistic sense of self intrudes into awareness. Depending on the balance, the result is "evil entities", gods, out-of-body experiences or distortions of space and time (Persinger 1993; Claim: Stated by an author, vendor or programme, without published supporting data.).
• 1994. The idea appeared in a case-study title: the sensed presence "as right hemispheric intrusions into the left hemispheric awareness of self" (Persinger et al. 1994; Anecdote: Personal reports, testimonials and single cases.).
• 2002. The lab described the mechanism as "intercalation between the hemispheres" (Persinger & Healey 2002). On this view the presence is, in effect, the person's own self perceived as someone else. That idea is close to the mainstream self/other account below, but Persinger tied it to hemispheres and to weak fields, and those two parts have not been independently supported.

In television segments Persinger described the classic lab report as a presence "usually on the left side" (TLC segment; Discovery Ultrascience).

Setting and dose

The details are in RESEARCH.md. In short:

• a blindfolded subject in an armchair, inside an acoustic chamber that was also a Faraday cage;
• subjects told it was a relaxation study, and asked to experience whatever happens without labelling it;
• mostly about 1 µT, from 100 nT up to about 10 µT, with almost no paper saying where the field was measured;
• 20 to 40 minutes per pattern, often a right-hemisphere pattern first, then a bilateral one.

The presence studies

What the lab said about rates

• "Up to 80 %." This is how the independent replicators summarised the lab's claims (Granqvist 2005). We found no single paper that reports 80 % with a sham comparison (Unverified: Nobody has tested it, or we could not check the source. as a figure).
• Visions of Christ. In an e-mail quoted by Murphy, Persinger wrote that about 20 of roughly 2,000 subjects had felt or seen Christ (Murphy: intro to the God Helmet; Anecdote: Personal reports, testimonials and single cases., second-hand).
• Sham levels. After the Swedish replication, Persinger and Koren wrote that sham groups score about 0.3 (SD 0.5) on the sensed-presence item and effective fields "0.8 or higher" (Persinger & Koren 2004; Claim: Stated by an author, vendor or programme, without published supporting data.). This is the clearest testable prediction the lab made.
• At home. Todd Murphy, who sells the consumer descendants of the lab helmet, writes that the presence "is less frequent" with his Shiva system than in the lab, and credits the lab's "rigorous conditions" (Murphy: God Helmet; Claim: Stated by an author, vendor or programme, without published supporting data.).

Problems in the record

• The human studies are small (6 to 71 subjects) and appeared mostly in two journals. No independent group has replicated one of the lab's presence effects (RESEARCH.md).
• In the lab's usage, "double-blind" meant blind to the hypothesis: subjects were told it was a relaxation study. Granqvist objected that nobody was blind to the condition (Granqvist, 27 Nov 2004).
• The 1997 button-press study pooled field and sham exposures in its headline figure.
• The physics is against it. Smooth signals from coils like these induce about a microvolt per metre in the brain. Neural networks start to respond at about 0.1–1 V/m (RESEARCH.md). Granqvist's group noted that the fields are about six orders of magnitude weaker than ordinary TMS (Granqvist 2005).

Independent tests and the suggestion explanation

Granqvist et al. 2005 (Uppsala)

• Design. Double-blind and sham-controlled, N = 89 (43 field, 46 sham). Equipment and software were lent by Persinger's lab. The Thomas pattern ran for 15 min over the right hemisphere, then burst-firing for 15 min over both, at 3–7 µT. The setting was a small isolation chamber with a blindfold (EXPERIMENTS.md).
• Predictors. Absorption, signs of temporal-lobe lability, and a "New Age" orientation, all measured before the session.
• Result. The authors found no evidence of any field effect, "neither in the entire group, nor in individuals high in suggestibility". The personality measures did predict sensed-presence and mystical reports (Granqvist 2005).
• Who had the strongest experiences. Two of the three people with strong spiritual experiences were in the sham group, as were 11 of the 22 with subtle ones (Khamsi 2004).
• Religious priming. In the same data, religiousness predicted mystical experiences with a religious quality. The authors read the pre-session questionnaires plus the sensory deprivation as a prime that activated religious schemas (Granqvist & Larsson 2006).

The dispute. Persinger and Koren said the timing was distorted on a fast Windows computer, that 15 min per pattern was too short, and that the Swedish field group's score (M = 0.3) showed they "never received an effective field configuration" (Persinger & Koren 2005). Granqvist replied that Persinger had specified both the duration and the software settings, and that the equipment was checked before and after (Larsson et al. 2005; archived letters in RESEARCH.md). The lab's re-analysis of 407 of its own subjects came a year later (St-Pierre & Persinger 2006). No third party has settled the timing and duration question.

French et al. 2009 ("The Haunt Project")

Seventy-nine people walked for 50 min in a dim circular chamber with hidden coils replaying Persinger's burst pattern, infrasound, both or neither. Neither the field nor the infrasound mattered. Only scores on a temporal-lobe-signs questionnaire predicted unusual reports. The study was not double-blind (French 2009).

Sham helmets

Three independent groups (Aarhus, Amsterdam, West Georgia) found that expectation, absorption and belief produce helmet experiences with no field. That part is Established: Replicated by independent groups.. It does not prove the field is inert; it proves that a striking experience under a helmet is not evidence that the field did anything.

Celebrity sessions

The well-known filmed sessions illustrate both sides, but they were unblinded single sessions (Anecdote: Personal reports, testimonials and single cases.):

• Richard Dawkins (BBC Horizon, "God on the Brain", 2003) reported dizziness, twitchy breathing and leg sensations but "didn't get the sensation of a presence" (clip). Persinger attributed this to Dawkins's low temporal-lobe sensitivity score (same clip).
• Michael Shermer sat in the dark for nearly an hour and reported a presence that "rushed by" him, which he was not sure was not himself (Skeptic). He knew what the session was meant to do.
• Susan Blackmore, a parapsychologist turned sceptic, tried the helmet in Persinger's lab and recalls it in a 2000 interview (Web of Stories).

What would settle it

RESEARCH.md describes the experiment that has not been run: Granqvist's blinding with Persinger's parameters (verified point timing, 30–40 min per pattern, a blindfold in a quiet chamber, the field checked before each subject), with the sensed-presence score as the primary outcome. Persinger's own figures (0.8 or more vs 0.3, SD 0.5) imply a large effect, about d = 1, so a modest sample would detect it (computed; EXPERIMENTS.md). This build's hardware-timed engine removes the "distorted timing" objection for the signals it plays.

Sensed presence without any helmet

Brain stimulation and lesions

• Arzy et al. 2006, Nature. A 22-year-old woman was being evaluated for epilepsy surgery and had no psychiatric history. Electrical stimulation of her left temporoparietal junction repeatedly produced a shadowy man behind her. He took her posture: when she leaned forward and hugged her knees, she felt him reach round and grasp her, which she found unpleasant (Arzy 2006; Nature news). The authors read it as the patient's own body, misplaced and not recognised as hers. Single lab: Reported by one laboratory or one group and its close collaborators, with no independent replication., one patient, but with milliamp currents applied directly to the cortex.
• Autoscopic phenomena. Peter Brugger and colleagues classified the feeling of a presence as one of a family of "doubles" of one's own body, alongside autoscopic hallucinations and heautoscopy (Brugger, Regard & Landis 1997).
• Blanke et al. 2014, Current Biology. Twelve neurological patients, mostly with epilepsy, who had felt a presence were compared on lesion maps. The presence was linked with sensorimotor loss and with damage to temporoparietal, insular and especially frontoparietal cortex (Blanke 2014). See the robot experiment below.

Robots and sensorimotor conflict

In the second part of the 2014 study, blindfolded healthy volunteers moved a master robot in front of them while a slave robot touched their backs.

• In sync, people felt they were touching their own back.
• With a delay between movement and touch, some felt a presence, located themselves further back, and estimated that more people were in the room. Some counted up to four, and some asked to stop (Blanke 2014; EPFL press release).

The authors concluded that the presence arises when the brain misattributes the source and identity of its own tactile, proprioceptive and motor signals. Blanke's group later used the same robots with Parkinson's patients. A subgroup was unusually sensitive to robot-induced presences, and a fronto-temporal network mapped in healthy people and lesion patients was disrupted in patients who had presences in daily life (Bernasconi 2021). This is one group across several cohorts (Single lab: Reported by one laboratory or one group and its close collaborators, with no independent replication.), but it is the only method that induces the experience in healthy people with a simple, physically plausible manipulation.

Extreme environments: the "third man"

Shackleton wrote of the 36-hour crossing of South Georgia in 1916 that it "seemed to me often that we were four, not three" (South, 1919). Worsley and Crean later told him they had felt the same. Similar accounts come from mountaineers, solo sailors, polar travellers, pilots and survivors of disasters. They were collected by Suedfeld and Mocellin (1987) and by John Geiger in The Third Man Factor (2009). In these accounts the presence is usually helpful: it guides, encourages or warns (Anecdote: Personal reports, testimonials and single cases., many independent reports).

The common conditions are monotony, isolation, cold, exhaustion, hypoxia, sleep loss and danger. In Brugger's interviews with eight extreme-altitude climbers, experiences clustered above 6,000 m and with solo climbing. They were unrelated to neuropsychological, EEG or MRI findings (Brugger 1999).

Sleep paralysis

During sleep paralysis a person wakes from REM sleep but cannot move. Cheyne's surveys found that hallucinations fall into an "Intruder" cluster (sensed presence, fear, voices and visions) and an "Incubus" cluster (chest pressure, breathing difficulty, pain) (Cheyne 1999).

• Fear was most clearly linked with the sensed presence (Cheyne, Newby-Clark & Rueffer 1999).
• Cheyne and Girard read the presence as an early output of a threat-activated vigilance system (Cheyne & Girard 2007).
• Nielsen proposed instead a spatial form of ordinary "social imagery", not tied to threat (Nielsen 2007).

A meta-analysis put the lifetime prevalence of the full "incubus" experience at about 11 % in random samples (Molendijk 2017). This matters for helmet sessions: Persinger's lab ran most sessions between midnight and 4 am, in darkness and silence, with relaxed subjects at the edge of sleep.

Bereavement

Rees interviewed 293 widowed people in Wales. Almost half had experienced their dead spouse, most often as a sense of presence, and most found it helpful (Rees 1971). Later reviews put post-bereavement experiences at 30–60 % of widowed people (Castelnovo 2015). A phenomenological study argues that many of these presences have no sensory quality at all, which is why "hallucination" fits them badly (Ratcliffe 2020).

Parkinson's disease

A feeling of presence is one of the "minor" hallucinations of Parkinson's disease, often an early one. In Fénelon's series it was usually brief and not distressing. It was linked with higher dopaminergic doses and with visual illusions or hallucinations (Fénelon 2000; Fénelon 2011). The robot work above suggests that it may mark a more severe course, with a higher risk of psychosis and cognitive decline (Bernasconi 2021).

Psychosis, anxiety and everyday life

Felt presence occurs in psychosis and paranoia, and also in people with no diagnosis. In Alderson-Day's three surveys, how often people felt a presence was predicted by a general tendency to hallucinate in every group, and by paranoia and being female in the population sample (Alderson-Day 2023). The Sensed Presence Questionnaire (SenPQ), which this app uses at enrollment, separates benign presences (protective, accompanying) from malign ones (threatening, at night) (Barnby & Bell 2017; QUESTIONNAIRES.md).

Proposed mechanisms

The mechanisms are compatible. Blanke's misattribution account and Persinger's "own self perceived as other" are close in spirit. They part company over what can trigger it: milliamp currents or sensorimotor conflict on one side, microtesla fields from outside the skull on the other.

Videos

Every link below was checked for title, channel and length. Timestamps were checked against the transcripts. Years are the broadcast or recording year where the video states it; otherwise the upload year (approximate, from YouTube's "years ago").

Documentaries and news

Scientists and authors

Murphy (vendor) talks

Sceptical overview

The full 2003 Horizon episode and a video of the 2014 robot experiment from EPFL itself were not found as verifiable uploads.

Reading list

Persinger laboratory

• Persinger, M. A. (1983). Religious and mystical experiences as artifacts of temporal lobe function. Perceptual and Motor Skills 57, 1255–1262. doi:10.2466/pms.1983.57.3f.1255
• Persinger, M. A. (1993). Vectorial cerebral hemisphericity as differential sources for the sensed presence, mystical experiences and religious conversions. Perceptual and Motor Skills 76, 915–930. doi:10.2466/pms.1993.76.3.915
• Persinger, Bureau, Peredery & Richards (1994). The sensed presence as right hemispheric intrusions into the left hemispheric awareness of self. Perceptual and Motor Skills 78, 999. doi:10.2466/pms.1994.78.3.999
• Cook & Persinger (1997). Experimental induction of the "sensed presence" in normal subjects and an exceptional subject. PubMed 9347559
• Cook & Persinger (2001). Experimental elicitation of the experience of a sentient being by right hemispheric, weak magnetic fields. PubMed 11361305
• Persinger, M. A. (2001). The neuropsychiatry of paranormal experiences. J Neuropsychiatry Clin Neurosci 13, 515–524. doi:10.1176/jnp.13.4.515
• Persinger & Healey (2002). Experimental facilitation of the sensed presence: possible intercalation between the hemispheres. J Nerv Ment Dis 190, 533–541. PubMed 12193838
• Tiller & Persinger (2002). Increased proportions of the left-sided sense of presence. PubMed 11883572
• St-Pierre & Persinger (2006). Re-analyses of 19 experiments. Int J Neurosci 116, 1079–1096. doi:10.1080/00207450600808800

Independent tests and sham helmets

• Granqvist et al. (2005). Sensed presence and mystical experiences are predicted by suggestibility, not by the application of transcranial weak complex magnetic fields. Neuroscience Letters 379, 1–6. doi:10.1016/j.neulet.2004.10.057
• Persinger & Koren (2005), reply, and Larsson et al. (2005), response. doi:10.1016/j.neulet.2005.03.060; doi:10.1016/j.neulet.2005.03.059
• Granqvist & Larsson (2006). Contribution of religiousness … in a sensory deprivation context. J Psychology 140, 319–327. doi:10.3200/JRLP.140.4.319-327
• French et al. (2009). The "Haunt" project: an attempt to build a "haunted" room by manipulating complex electromagnetic fields and infrasound. Cortex 45, 619–629. doi:10.1016/j.cortex.2007.10.011; accepted manuscript
• Andersen et al. (2014). Mystical experience in the lab. Method & Theory in the Study of Religion. doi:10.1163/15700682-12341323
• Maij & van Elk (2018). Getting absorbed in experimentally induced extraordinary experiences. Consciousness and Cognition 66, 1–16. doi:10.1016/j.concog.2018.09.010
• Maij, van Elk & Schjoedt (2019). The role of alcohol in expectancy-driven mystical experiences. Open full text

Mainstream presence research

• Barnby, Park, Baxter, Rosen, Brugger & Alderson-Day (2023). The felt-presence experience: from cognition to the clinic. Lancet Psychiatry 10, 352–362. doi:10.1016/S2215-0366(23)00034-2
• Alderson-Day et al. (2023). Varieties of felt presence? Three surveys of presence phenomena and their relations to psychopathology. Psychological Medicine 53, 3692–3700 (open access). doi:10.1017/S0033291722000344
• Alderson-Day, B. (2023). Presence: The Strange Science and True Stories of the Unseen Other. Manchester University Press (book).
• Barnby & Bell (2017). The Sensed Presence Questionnaire (SenPQ). PeerJ 5, e3149. doi:10.7717/peerj.3149
• Arzy, Seeck, Ortigue, Spinelli & Blanke (2006). Induction of an illusory shadow person. Nature 443, 287. doi:10.1038/443287a
• Blanke et al. (2014). Neurological and robot-controlled induction of an apparition. Current Biology 24, 2681–2686. doi:10.1016/j.cub.2014.09.049
• Bernasconi et al. (2021). Robot-induced hallucinations in Parkinson's disease depend on altered sensorimotor processing in fronto-temporal network. Science Translational Medicine 13, eabc8362. doi:10.1126/scitranslmed.abc8362
• Brugger, Regard & Landis (1997). Illusory reduplication of one's own body. Cognitive Neuropsychiatry 2, 19–38. doi:10.1080/135468097396397
• Brugger et al. (1999). Hallucinatory experiences in extreme-altitude climbers. PubMed 10082335
• Suedfeld & Mocellin (1987). The "sensed presence" in unusual environments. Environment and Behavior 19, 33–52. doi:10.1177/0013916587191002
• Geiger, J. (2009). The Third Man Factor: Surviving the Impossible. Penguin Canada / Weinstein Books (book).
• Shackleton, E. (1919). South. Project Gutenberg

Sleep paralysis, bereavement, Parkinson's

• Cheyne, Rueffer & Newby-Clark (1999). Hypnagogic and hypnopompic hallucinations during sleep paralysis. Consciousness and Cognition 8, 319–337. doi:10.1006/ccog.1999.0404
• Cheyne, Newby-Clark & Rueffer (1999). Relations among hypnagogic and hypnopompic experiences associated with sleep paralysis. J Sleep Research 8, 313–317. doi:10.1046/j.1365-2869.1999.00165.x
• Cheyne & Girard (2007). Paranoid delusions and threatening hallucinations. Consciousness and Cognition 16, 959–974. doi:10.1016/j.concog.2007.01.002
• Nielsen (2007). Felt presence: paranoid delusion or hallucinatory social imagery? Consciousness and Cognition 16, 975–983. doi:10.1016/j.concog.2007.02.002
• Sharpless & Barber (2011). Lifetime prevalence rates of sleep paralysis. Sleep Medicine Reviews 15, 311–315. doi:10.1016/j.smrv.2011.01.007
• Molendijk et al. (2017). Prevalence rates of the incubus phenomenon. Frontiers in Psychiatry 8, 253. doi:10.3389/fpsyt.2017.00253
• Rees, W. D. (1971). The hallucinations of widowhood. BMJ 4, 37–41. doi:10.1136/bmj.4.5778.37
• Castelnovo et al. (2015). Post-bereavement hallucinatory experiences. J Affective Disorders 186, 266–274. doi:10.1016/j.jad.2015.07.032
• Ratcliffe (2020). Sensed presence without sensory qualities. Phenomenology and the Cognitive Sciences. doi:10.1007/s11097-020-09666-2
• Fénelon et al. (2000). Hallucinations in Parkinson's disease: prevalence, phenomenology and risk factors. Brain 123, 733–745. doi:10.1093/brain/123.4.733
• Fénelon et al. (2011). Feeling of presence in Parkinson's disease. JNNP 82, 1219–1224. doi:10.1136/jnnp.2010.234799; author manuscript

Trying it with this helmet

What to expect

Be clear about two things before you start.

• The field is weak, and weaker where the lab said it mattered. Programs default to 1 µT, measured 20 mm in front of each coil, which is roughly the cortex under it. Less than 0.1 µT reaches the depth of the amygdala and hippocampus. Full current gives about 8 µT at 20 mm (RESEARCH.md). The temporoparietal cortex that the presence programs aim at is near the surface. The Brain tab of each program shows the computed field (BRAIN_VIEW.md).
• The independent replication was null. A presence during a session is quite likely in a dark, silent room, especially if you are absorbed, expectant or close to sleep. On its own that says nothing about the field. Only a real-vs-sham difference under a blind that holds does.

Programs

The lab never published its point values for burst-firing. The programs use the published timing with a stand-in burst; the Thomas pattern is the exact patent table. Each program's notes say what was copied and what was inferred (SIGNALS.md).

Setting

Copy the lab's setting as closely as you can, and keep it identical in every session:

• a dark, silent room ("silence, not just quiet", in Murphy's phrase); a blindfold or opaque goggles; seated or reclined;
• the same time of day each session. The lab worked mostly at night, but a sleepy subject brings sleep-onset presences with them, so note how drowsy you were;
• neutral instructions. Persinger's subjects were told to experience whatever happens without labelling it; Granqvist's were told the study concerned the effect of weak fields on experiences. Write your wording down and reuse it;
• no meditation, other devices, alcohol or drugs.

A blinded design

Follow EXPERIMENTS.md. The short version:

1. Enrol first. Run Guides → Enrollment battery (one time) on an earlier day. It includes the SenPQ, so your baseline tendency to sense presences is on record (QUESTIONNAIRES.md).
2. Check the blind. Do the cue test with the helmet on a cushion, not a person: record sound and vibration with a phone, then do a 20-run listening test. If a listener can tell real from sham, fix that before anything else.
3. Pick the comparison and write it down.
   • The simplest: presence-right vs sham. Guides → Blinded helmet session with questionnaires runs exactly this, with a free report, the post-session checklist and a real/sham guess before the reveal (GUIDES.md).
   • The Healey design: right vs left vs sham, in blinded, counterbalanced order (a helper chooses right or left; tick Blind each time).
   • The duration test: persinger-saroka-2015-sensed-presence or god-helmet-classic vs sham. This tests Persinger's claim that Granqvist's 15-min phases were too short.
4. Rate before you look. Write the free report first. Then rate the presence item 0–10, and note where the presence was (left, right, behind, in front), whether it felt like a known person, its emotional tone, and roughly when it came. Then make your real/sham guess.
5. Count every session. Persinger's own figures imply a large effect, about d = 1; detecting it needs about 17 sessions per arm, and a medium effect about 64 (computed; EXPERIMENTS.md). At one session per 72 hours, a solo series takes months. A group with one session per person is faster.

A real field effect would show up as higher presence ratings in the real arm than in the sham arm, with a blinding index near zero. Several vivid presences in a series that also has vivid presences under sham are what the independent literature predicts.

Safety

Read SAFETY.md first. The points that matter most here:

• Right-sided fields frightened some lab subjects, by Murphy's account. In the classic program, phase 1 (right side only) can cause fear; Skip jumps to phase 2. Stop at once on fear, panic, headache, nausea or dizziness, and don't repeat a session that felt bad.
• The app raises a safety alert when the post-session checklist shows a sensed presence of 5 or more together with anxiety of 5 or more (QUESTIONNAIRES.md).
• Do not use the helmet with epilepsy or a seizure history, a psychiatric diagnosis or psychiatric medication, implanted electronics, pregnancy, a recent head injury, or under 18. Nightly frightening presences, sleep paralysis that distresses you, or presences with other new symptoms are a reason to see a doctor, not a reason to run sessions. In Parkinson's disease a new feeling of presence is worth reporting to the neurologist.
• At most one session per 72 hours (Murphy recommends weekly, with a 21-day break after six). Have someone else present for your first session. Don't drive for an hour afterwards.
• If you run sessions on other people, screen them, debrief them, and never deceive them about risk (SAFETY.md).