Your lip line and tooth display at rest describe what your smile looks like before you smile, and it is a separate record from the one taken when you grin. Dentists look at both because they answer different questions. What shows when your face is relaxed tells us about lip length, lip position and where the biting edges of your front teeth sit. What shows in a full smile tells us how far your lip travels. Neither one on its own is enough.
Key takeaways
- Display at rest is recorded with the facial muscles relaxed and the teeth slightly apart, which is a different position from simply putting the lips together.
- Vig and Brundo reported in 1978 that upper front tooth display at rest falls steadily with age, while lower incisor display rises.
- Misch found in 2008 that the canine tip is a far more stable landmark than the central incisor, with over 90 per cent of canine averages within 1 mm regardless of age or sex.
- Jeelani and colleagues found in 2018 that upper lip length was the strongest single predictor of incisor display, explaining 29.7 per cent of the variance.
- A high lip line and excessive gingival display are not the same thing, and Sabri and StatPearls both document cases where one occurs without the other.
- Wide variation between people is normal, and Yong and colleagues showed in 2022 that the familiar 2 mm guide only suits one lip form.
What this article is, and what it is not
This piece is about the lip and the resting position. Gummy smile causes and gummy smile treatment are covered separately in our article on gummy smile causes and treatment, and nothing about treating gum display appears here. If that is what brought you, go there instead. This page is about why your dentist photographs your face with your lips relaxed, and why the amount of tooth you show has changed over the years.
Two related pages may also help. Our article on why a smile looks off centre covers the midline, and buccal corridors and why a smile looks narrow covers the dark spaces at the corners. Both are about the smiling position rather than repose.
Repose is a coached position, not just lips together
Repose means the perioral muscles are unstrained, the jaw is relaxed and the teeth are slightly apart. It is not the same as bringing your lips together deliberately, and the difference is measurable. Burstone reported in 1967, as summarised by Zachrisson, that the gap between the lips in adolescents was 1.8 mm in centric occlusion but 3.7 mm at rest. Two records, two numbers, same person.
Because repose is easy to lose the moment someone points a camera at you, clinicians use phonetic cues. Misch asked patients to say "emma" and then relax the face and lower jaw. Ackerman and Ackerman, in the Journal of Clinical Orthodontics in 2002, had patients say "Chelsea eats cheesecake on the Chesapeake", relax, then smile, captured on video so head position was controlled. Drummond and Capelli added the "M" phoneme to capture the least incisor exposure, citing Morley and Eubank. Zachrisson's advice in 1998 was to coach the patient until they can reproduce the same lip position twice in succession before recording anything, and to prefer a short video over a single still.
How much tooth shows at rest
The most-quoted figures come from Vig and Brundo, in the Journal of Prosthetic Dentistry in 1978. As reproduced in later texts, they reported maxillary central incisor exposure at rest of 1.91 mm in men and 3.40 mm in women, with a steep age gradient: 3.31 mm under 29, 1.58 mm at 30 to 39, 0.95 mm at 40 to 49, 0.46 mm at 50 to 59 and 0.04 mm above 60. A short upper lip of 10 to 15 mm exposed 3.92 mm of tooth; a long lip of 31 to 35 mm exposed 0.25 mm.
Those numbers deserve a caveat, and Misch supplied it in the Journal of Prosthodontics in 2008, criticising the 1978 paper for not reporting the number of patients, the method of evaluation or the range of exposure. Treat them as historically reported averages rather than a modern norm, and never quote the headline figure without the age qualifier. An average of 2 to 4 mm sits on top of a dataset that also recorded 0.04 mm in people over 60.
Misch's own study measured 104 dentate patients aged 30 to 59 clinically with a ruler. Female central incisor exposure at rest averaged 3.8 mm with a range of minus 1 to plus 8 mm, and male exposure averaged 2.5 mm with a range of minus 3 to plus 7 mm. By decade, women showed 4.1, 2.8 and 1.8 mm across their thirties, forties and fifties, and men 3.2, 2.4 and 1.4 mm. His conclusion was blunt: the average figure represented less than 20 per cent of his subjects and could not be used predictably.
The more interesting finding in that paper is the canine. Female canine tip display averaged 0 mm with a range of minus 2 to plus 2 mm, and male minus 0.5 mm with a range of minus 3 to plus 2 mm, and over 90 per cent of canine averages fell within 1 mm regardless of age or sex. The canine is the stable landmark, not the central incisor. That is the opposite of what most people assume.
Jeelani, Fida and Shaikh, in the Dental Press Journal of Orthodontics in 2018, measured 150 patients aged 18 to 30 and found males showing 3.72 mm and females 4.77 mm at rest. Their more useful result was what drives it. Upper lip length was the strongest single predictor at 29.7 per cent of the variance, followed by upper lip thickness at 27.3 per cent and mandibular plane angle at 25.8 per cent. Incisor inclination explained only 3.2 per cent. How much tooth shows at rest is mostly a lip question, not a tooth question.
What changes over a lifetime, and in both directions
Everyone knows the upper teeth show less as the years pass. Almost nobody knows the lower teeth show more. Zachrisson, summarising Vig and Brundo in 1998, put it memorably: mandibular incisor display after age 60 is approximately equal to maxillary incisor display before age 30. Vig and Brundo also reported the sex pattern running the other way for the lower teeth, with men at 1.2 mm and women at 0.5 mm at rest.
Drummond and Capelli's 2016 study in the Angle Orthodontist used standardised videography with 265 participants aged 19 to 60 and found the same pattern. Maxillary incisor display decreased significantly with age at rest, in speech and in smile, more markedly in men. Mandibular display during speech increased with age, and was the only variable with no significant sex difference. Upper lip length increased by 2.25 mm in men and 0.49 mm in women between 19 and 60.
Van der Geld, Oosterveld and Kuijpers-Jagtman, in the European Journal of Orthodontics in 2008, followed 122 males across three age cohorts and found the same. Maxillary lip line heights decreased in all functions, the smiling lip line reduced by about 2 mm, mandibular display at rest increased, and upper lip length increased by almost 4 mm while upper lip elevation did not change. The lip got longer, not weaker.
This is soft tissue ageing, not teeth moving or sinking. Gravity, flattening and loss of skin elasticity change where the lip sits. The teeth stay where they are.
One separate change can work against that. If the gum margin migrates, the visible part of the tooth gets longer even though the tooth itself has not moved, so a person can show less upper tooth at rest and still have longer looking teeth when they do smile. That is a gum level change rather than a lip one, and it is covered in our guide to receding gums, their causes and treatment.
Upper lip length, and why the absolute number matters less than you would think
Sabri, in the Journal of Clinical Orthodontics in 2005, compiled average upper lip length at rest from five earlier sources and gave roughly 23 mm in males and 20 mm in females, measured from subnasale to the lowest midline point of the upper lip. StatPearls independently gives 20 to 24 mm in young adults, rising with age, and describes under 20 mm as a short lip. Those figures come from different methods across decades, so treat them as a range rather than a target.
Sabri's better observation was relational: absolute lip length matters less than lip length relative to commissure height, and the two should be roughly equal. Peck, Peck and Kataja, in the American Journal of Orthodontics and Dentofacial Orthopedics in 1992, found that males had significantly longer upper lips than females, and that at maximum smile the female upper lip sat on average 1.5 mm more superiorly than the male. That single measurement is the origin of the widely repeated claim that female lip lines sit about 1.5 mm higher.
Lip mobility: the variable that connects the two records
Spear set out the arithmetic plainly: display at rest plus lip mobility equals smile display. That is why display at rest alone cannot determine where an incisal edge should sit, and why a full smile photograph on its own tells you very little about the resting position.
You will see mobility figures quoted everywhere, usually 2 to 12 mm with an average of 7 to 8 mm, often with a claim that anything above 8 or 10 mm counts as a hypermobile lip. These are widely repeated in clinical teaching, but the trail leads to a 1997 conference course rather than a published measurement study, so treat them as conventions rather than findings. What is reliably reported is Zachrisson's reason for measuring at rest in the first place: the full smile will not provide the same information, because of the high individual variability in how far the upper lip travels.
A high lip line is not the same as a gummy smile
A high lip line is a classification of where the upper lip sits at full smile relative to the crowns. Tjan, Miller and The, in the Journal of Prosthetic Dentistry in 1984, assessed 454 full-face photographs of dental and dental hygiene students aged 20 to 30, scored by visual judgement rather than measurement. A high smile showed the full length of the upper front teeth plus a contiguous band of gum, an average smile 75 to 100 per cent of the teeth plus the interproximal gum only, and a low smile less than 75 per cent. They found 10.57 per cent high, 68.94 per cent average and 20.48 per cent low, with high smiles in 13.79 per cent of women against 6.76 per cent of men, and low smiles in 29.95 per cent of men against 12.50 per cent of women. Tjan himself warned that the criteria should not be treated as rules, and this was a group of students in Los Angeles in 1984, not an Australian population sample.
Excessive gingival display is a different construct: it is a quantified amount of gum, and there is no agreed threshold. Zachrisson used 2 mm or more. StatPearls notes that some authors define a gummy smile as more than 3 to 4 mm while others consider more than 2 mm excessive, and that 1 to 2 mm on smiling is generally considered normal.
The two dissociate in both directions, and this is documented. Sabri published a case with a high lip line at rest but no gum showing on smiling, because the lip was hypomobile. StatPearls records the reverse: patients with hyperactive lip elevator muscles can have a normal incisor display at rest and still show excessive gum when they smile.
One more myth worth retiring. A short upper lip is commonly blamed for gum display, but Peck, Peck and Kataja found that upper lip length, clinical crown height and the mandibular and palatal plane angles did not appear to relate to the gingival smile line at all. Their finding pointed instead to anterior vertical maxillary excess combined with increased muscular capacity to raise the lip. The full picture of causes is in our gummy smile article.
Why a posed photo can mislead
Ackerman and Ackerman named two problems with conventional still photography. The first is camera orientation: retracted intraoral frontals are shot perpendicular to the occlusal plane while smile photographs are shot perpendicular to the face in natural head position, so the smile arc looks different in the two views. The second is repeatability: in their words, it is impossible to repeat the social smile exactly during one photography session, let alone over a longer period. They also noted that lighting alone can change the apparent buccal corridor.
Misch flagged a limitation of his own: when the canine sat above the resting lip, the lip had to be slightly distorted to measure it, and passive measurements would not have allowed that assessment at all. A clinical look is not the same as a photograph. If you have wondered why a smile can look crooked in one image and level in another, our article on facial asymmetry and why a smile looks crooked covers that separately.
Variation between people is the normal finding
Owens, Goodacre and colleagues, in the International Journal of Prosthodontics in 2002, studied 253 subjects across six groups and found women displayed significantly more gum in four of the six. Those datasets are small or dated, Misch's sample was Caucasian by design and he wrote that further studies were needed, and none of these averages should be applied as an expectation for an individual.
Yong and colleagues, in the American Journal of Orthodontics and Dentofacial Orthopedics in 2022, asked 107 respondents, roughly half of them professionals, to rate manipulated images across straight, moderate and high lip forms. The familiar 2 mm guide suited only straight lip form. Moderate and high lip forms were preferred with 4 to 5 mm of incisal show, and professionals and laypeople largely agreed. There is no single ideal number.
| What is being looked at | With the lips at rest (repose) | In a full smile | Why the difference matters |
|---|---|---|---|
| What the dentist is recording | Where the upper lip sits when the muscles are relaxed and the teeth are slightly apart | Where the upper lip travels to when the lip elevator muscles contract | Two muscle states, one passive and one active |
| Upper front tooth show | Typically a few millimetres in younger adults, commonly reducing toward zero with age (Vig and Brundo 1978; Misch 2008) | Usually most or all of the crown length, in around 69 per cent of young adults (Tjan and colleagues 1984) | Nearly everyone shows upper teeth when smiling, even if none show at rest |
| Lower front tooth show | Increases with age in both men and women (Vig and Brundo 1978; Drummond and Capelli 2016) | Largely masked by the lower lip | The at-rest record is the only place this age change is visible |
| Upper lip length | Measured subnasale to stomion; lengthens with age, more so in men (Drummond and Capelli 2016) | Shortens as it stretches laterally and elevates | Explains why the same person's display changes over decades |
| The vocabulary used | Incisal display at rest, or lip-tooth relationship at repose | Smile line or lip line, described as high, average or low (Tjan and colleagues 1984) | Separate descriptions of separate records, not synonyms |
| Gum visibility | Gum is not normally visible in repose | Gum may or may not be visible depending on how far the lip travels | A high lip line describes lip position; excessive gingival display describes an amount of gum |
| How it is captured | A relaxed, coached position, often cued with an "M" or "emma" sound (Zachrisson 1998; Misch 2008) | A posed social smile, hard to repeat exactly; video captures it better than a still (Ackerman and Ackerman 2002) | A single posed photo can misrepresent both records |
| What it informs | Denture and implant tooth position, incisal edge position, orthodontic vertical planning, resting lip seal | Gum display, smile arc, how much of the arch is framed by the lips | The gap between the two records is lip mobility |
Frequently asked questions
How much of your front teeth should show when your mouth is at rest?
There is no single correct number. Historically reported averages sit in the low millimetres for adults, but Misch found a range from minus 3 to plus 8 mm and concluded the average could not be used predictably, and Yong and colleagues found that 4 to 5 mm suited moderate and high lip forms better than the usual 2 mm guide. Age matters more than almost anything else.
Why do my teeth show less than they used to?
Because the upper lip lengthens. Van der Geld and colleagues measured an increase of almost 4 mm in upper lip length across cohorts aged 20 to 25 and 50 to 55, with no change in how far the lip elevated. The teeth are not sinking or shortening; more of them is being covered.
What is a lip line in dentistry?
A lip line, or smile line, describes where the upper lip sits at full smile relative to the front teeth. Tjan and colleagues classified it as high, average or low depending on how much of the crown and gum is visible. It is a description of lip position, not a diagnosis.
Is a high lip line the same as a gummy smile?
No. A high lip line describes where the lip sits; excessive gingival display describes an amount of gum, and there is no agreed threshold for the latter. Sabri documented a high lip line with no gum showing because of a hypomobile lip, and StatPearls records the reverse. They are related but they are not synonyms.
Why does my dentist take a photo of my face with my lips relaxed?
Because repose is a separate record. It shows where the biting edges of the upper front teeth sit relative to a relaxed lip, which informs incisal edge position, denture and implant tooth position, and orthodontic vertical planning. A smiling photograph cannot substitute, because how far a lip travels varies enormously between people.
What is lip mobility and why does my dentist measure it?
Lip mobility is how far the upper lip travels between repose and a full smile. It matters because display at rest plus lip mobility equals smile display, so the same resting position can produce very different smiles in two people. Specific cut-offs quoted for hypermobility come from clinical teaching rather than published measurement.
If you would like your smile assessed properly
The team at Lumi Dental in Melrose Park records both repose and a full smile as part of a cosmetic assessment, and can talk through what is normal variation and what, if anything, is worth discussing further. You can book a complimentary cosmetic consultation to have that conversation without any obligation, and you are welcome to check our current offers page as well. We are open Monday to Saturday.
This article is general information only and is not a substitute for an examination and individual advice from a registered dental practitioner. Individual results and suitability vary, and the measurements described here are population averages rather than targets for any one person.
