Which teeth are the molars? And why is the second molar the one people ask about?

Which teeth are the molars? And why is the second molar the one people ask about?

Direct answer (60 characters or fewer in the source language)

Count back from the midline in each quadrant: teeth 4 and 5 are the premolars, teeth 6, 7 and 8 are the molars [F8][F9]. The second molar is the one whose number ends in 7 [F8]. No literature ranks teeth by importance [F22].

Scope: this is general health education based on international literature and an international standard. It does not cover any particular country's insurance scheme or regulations; for how care and fees are organised, follow the rules where you are.

First, which "molar" are you asking about

This card answers a question that people ask in Chinese and Japanese, where one everyday word (臼齒 / 臼歯) covers two different categories of tooth at once. English separates them - premolar and molar - but the same ambiguity comes back the moment someone says "my back tooth hurts". In the literature and in clinical records the two are separate classes: premolars and molars, and the permanent dentition is classified into four classes - incisors, canines, premolars and molars [F2]. So "my back tooth hurts" may mean the premolars, whose numbers end in 4 or 5, or the molars, whose numbers end in 6, 7 or 8. Identify the tooth first; everything after that depends on it.

What this card covers is what these teeth are, how they are counted, and what they do in occlusion. Whether a molar needs replacing after extraction, and what the options are, belongs to a different card (KM-DENTAL-34, linked at the end). This card does not rewrite that content and only cites it where needed [F22].

1. There are 32 permanent teeth, 8 in each quadrant

• A person has two sets of teeth: 20 deciduous teeth and 32 permanent teeth, 16 in the maxilla and 16 in the mandible. The permanent teeth are classified as incisors, canines, premolars and molars, and the primary teeth include no premolars [F2].
• The primary incisors, canines and molars are replaced at the change of dentition by the permanent incisors, canines and premolars respectively; the three permanent molars are additional positions rather than replacements for any primary tooth [F3]. This explains a common misunderstanding: the big tooth that appears at the back of the arch at around age 6 has not replaced a primary tooth (what sheds at that time is a primary incisor at the front), and it is a permanent tooth from the moment it appears - there is no second chance at it [F3][F4].
• Divide the 32 evenly into four quadrants and each quadrant has 8. Following the classes and the eruption list above, counting back from the midline the order is: central incisor, lateral incisor, canine, two premolars, three molars [F3][F4]. That "how many teeth back from the midline" order is exactly what tooth numbering is built on [F8][F22].

2. How to read a tooth number

Why the same tooth carries different numbers

Three notation systems have coexisted worldwide for a long time: the Zsigmondy/Palmer system, the universal system, and the FDI two-digit system [F10][F11]. All three remain in use, and confusion follows from that - a study proposing a new notation surveyed 66 dental personnel and recorded the confusion that exists among the systems currently in use [F12]. So when a number on a chart, a quotation or a referral letter does not match one you have seen elsewhere, asking "which system is this" is usually faster than trying to convert.

The FDI two-digit system (the international standard)

• This system was originally drawn up by the FDI World Dental Federation and approved by the World Health Organization and by ISO Technical Committee 106, becoming the international standard ISO 3950. What the standard provides is a system for designating teeth or areas of the oral cavity using two digits; the current edition is the fourth, published in 2016, which cancels and replaces the third edition of 2009 [F7].
• How the two digits split: the first digit is the quadrant and the second is "how many teeth back from the midline", with the quadrants running upper right, upper left, lower left, lower right, and tooth positions counted 1 to 8 from the midline backwards [F8]. Several sources describe the same rule consistently: the first digit indicates the quadrant, the second the tooth within that quadrant, counted from the midline posteriorly [F9].
• So numbers ending in 6, 7 and 8 are the three molars, and those ending in 4 and 5 are the two premolars. For example, 37 means the lower left quadrant, seventh from the midline - the lower left second molar; the symmetrical tooth on the lower right is written 47 [F22].
• Primary teeth also use two digits, but with a different set of quadrant digits. For the upper right region, for instance, permanent teeth start with 1 while primary teeth start with 5 (primary teeth 54 and 55 are the upper right primary molar region) [F23].

The universal numbering system (1 to 32)

The universal system does not use quadrants; it runs one continuous sequence, beginning with the upper right wisdom tooth (number 1), counting clockwise around the dentition and finishing with the lower right wisdom tooth (number 32) [F9]. Put another way, the permanent maxillary teeth are numbered 1 through 16 from right to left, and the permanent mandibular teeth 17 through 32 from left to right [F5]. Applying that rule, the upper right second molar is number 2 - the same tooth that FDI writes as 17. One tooth, two systems, completely different notation [F22]. Geographically, the FDI two-digit system is widely used throughout the world except the USA [F11], while the US textbook entry uses the universal numbering as its main frame of description [F5].

The Zsigmondy/Palmer system

This is the earliest of the three: permanent teeth are numbered from the central incisor (1) back to the third molar (8), primary teeth use Roman numerals or letters, and a quadrant symbol marks which region the tooth is in [F9]. Comparing the rules as described in sources [F8] and [F9], its digit sequence is the same as the FDI second digit; the difference is that the quadrant is shown by a symbol rather than a digit [F22].

Three things you can do when you are handed a number

1. Ask which notation is being used (two digits, a continuous 1 to 32, or a quadrant symbol) [F10][F22].
2. Ask the dentist to point at the tooth on the radiograph or intraoral photograph - a position is harder to misread than a number [F22].
3. Write the number down together with the location (for example "37, lower left second molar"), so that nothing is lost in handover if you change clinics [F22].

3. What premolars and molars each do

• Teeth as a whole provide a masticatory system that functions in the incising, tearing and grinding of food [F6]; the four classes each have their function according to structure and position [F2]. This card does not assign "which class does which action" tooth by tooth - the sources obtained for this card do not go to that level of correspondence, so it is not written [F22]. What premolars and molars have in common is that they sit at the back of the arch, which is also where occlusal force is more heavily distributed (see the next point) [F15].
• Occlusal force is not distributed evenly. A study that recorded the variables in 16 healthy young adults using an occlusal pressure measurement system found that the distribution of occlusal force was greatest at the molar region, followed by the premolar region and then the anterior region; and that the proportion carried by the molar regions rose as clenching strength increased [F15]. This is a small-sample laboratory measurement: the direction is informative, but the individual values should not be extrapolated to any one person.
• "How many pairs of back teeth still meet" is closer to chewing ability than "how many teeth are left in total". A systematic review pooled subjects with 3 to 5 posterior occluding pairs together with those having 0 to 2, and recorded that their comminution or mixing ability was 28% to 39% lower than that of subjects with complete dentitions [F16]. The full discussion of replacing or not replacing a missing tooth belongs to card KM-DENTAL-34 and is not repeated here.
• The World Health Organization considers the number of teeth to be a key indicator of oral health status; the textbook entry records that retention of function, aesthetics and a natural dentition of at least 20 teeth with nine to ten occluding pairs (including anterior teeth) is associated with adequate masticatory efficiency and ability [F6]. Note that this sentence is about the dentition as a whole, not about the importance of any single tooth, and that the source states an association, not causation.

4. At what ages the three molars erupt

• The eruption ranges recorded in the textbook entry: the permanent first molars erupt between 6 and 7 years of age, the permanent second molars between 11 and 13 years, and the permanent third molars (wisdom teeth) between 17 and 21 years, and then only if there is room for them [F4].
• Eruption timing differs markedly between populations. A systematic review and meta-analysis of 80 studies covering Asia, Europe, Africa, North America and Oceania recorded that mandibular first molar eruption was found to be as early as 4.09 years, while the maxillary second molar erupted as late as 13.45 years; meta-regression interpreted eruption to be earlier in females and in the mandible [F13].
• Calculated from a nationally representative US survey: 48% of 6-year-olds had all four first permanent molars emerged, rising to 98% at age 8, whereas second permanent molar emergence varied more [F14].
• So "the age at which it should come through" is a range for reference only. A child whose teeth arrive a little earlier or later than peers is not thereby abnormal; where there is doubt, a dentist decides on intraoral examination and imaging.

5. Why the second molar is the one that keeps coming up

To say it plainly first: none of the literature found for this card ranks teeth in order of importance. What follows are verifiable facts that explain why this particular tooth so often becomes the focus of the question [F22].

• It is often the last functionally occluding tooth at the back of the arch. Wisdom teeth are among the permanent teeth most often congenitally absent: an updated systematic review and meta-analysis published online on 11 June 2026 (125 data points, 87,282 individuals) calculated a pooled global prevalence of third molar agenesis - meaning the tooth never formed, as distinct from one that was extracted or lost - of 23.07% (95% CI 21.2 to 25.0%), and recorded substantial heterogeneity across studies [F20]. On a side where the wisdom tooth is absent, the second molar is the functional terminal tooth of that side [F22].
• It sits next to the wisdom tooth, so the wisdom tooth's position affects it. A 2025 systematic review and meta-analysis of 13 studies and 13,788 patients calculated that, where a mandibular second molar is adjacent to an impacted mandibular third molar, the pooled prevalence of caries on its distal surface (the side facing the wisdom tooth) was 29.89% (95% CI 21.05 to 38.74%); by Winter's classification, the mesioangular position was the one most frequently associated with caries, at 43.37% [F17].
• But "high prevalence" is not "it will certainly decay". A separate systematic review looking specifically for incidence rather than prevalence screened 81 records and was able to include only 2 cohort studies; it states that the evidence is insufficient and that higher-quality research is still required. Both studies indicated that distal surface caries occurred more when third molars were erupted in the intermediate term, and more over the long term in an ageing male population [F18]. Cross-sectional prevalence figures can show that something is common; on their own they cannot establish causation or predict an individual.
• Whether to remove a wisdom tooth prophylactically in order to protect the second molar has no settled answer in the literature. The Cochrane systematic review (current version updated in 2020) concludes that insufficient evidence is available to determine whether asymptomatic disease-free impacted wisdom teeth should be removed or retained; retention may be associated with an increased risk of periodontitis affecting adjacent second molars in the long term, but that evidence is of very low certainty. Given the lack of evidence, the authors advise that patient values be considered and clinical expertise used to guide shared decision-making, with periodic clinical assessment if the tooth is retained [F19].
• What happens after extraction, and whether to replace the tooth, is the subject of another card (KM-DENTAL-34), which is organised around over-eruption of the opposing tooth, tipping of neighbouring teeth, the shortened dental arch concept, and the survival rates of the various replacement options. This card does not rewrite it [F22].

6. Risk factors and limits

• The pit-and-fissure structure of the molar occlusal surface makes cleaning harder, and it is one of the sites where caries commonly starts. A Cochrane systematic review (38 trials, 7,924 children in total; an update of the 2004, 2008 and 2013 versions) records that, compared with no sealant, resin-based sealants corresponded to a lower rate of caries in first permanent molars - the group of molars whose numbers end in 6 - in children aged 5 to 10 years; that estimate comes from 7 of those trials, OR 0.12 at 24 months follow-up, 95% CI 0.08 to 0.19, moderate-quality evidence [F21]. Note that the evidence population is limited to children and the site to that one group of molars; whether it applies to you or to your child is for a dentist to judge.
• The risk around an impacted wisdom tooth runs in both directions. Retention may relate to long-term periodontal risk for the neighbouring tooth (very low certainty evidence), while removal is surgery with its own indications and complications; the Cochrane review states plainly that the evidence is insufficient to generalise [F19][F17].
• Every proportion and interval cited in this card is a group-level figure from research, reflecting population averages. They cannot be used to estimate any individual's outcome; the populations, ages, radiographic reading criteria and follow-up lengths differ between studies [F17][F18][F20].
• A tooth number is not a diagnosis. The number states a position, not a condition; whether a given tooth needs treatment, and which treatment, must be assessed by a dentist through clinical examination and imaging.
• Wisdom tooth removal, caries restoration and replacement of missing teeth carry risks and contraindications; the actual treatment method and its results vary from person to person and must be assessed by a dentist.

7. Before your appointment: a 7-question checklist

1. The tooth I am worried about - what is its number, and which notation is being used (two digits, a continuous 1 to 32, or a quadrant symbol)?
2. Is it a molar ending in 6, 7 or 8, or is it actually a premolar ending in 4 or 5? Could you point it out to me on the image?
3. What angle is my wisdom tooth (ending in 8) lying at? Can I reach the gap between it and the second molar next to it when I brush?
4. Is there caries on the side of my second molar that faces the wisdom tooth (the distal surface)? Can that be read on the image?
5. How many pairs of back teeth still meet on each side at the moment?
6. If this is about a child: have the molars ending in 6 and 7 come through? Given their age and caries risk, what preventive measures are called for?
7. If you are recommending treatment or extraction, what is the indication? What alternatives are there? And how often should I come back afterwards?

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Compliance notice

This is health education information under Article 87 of Taiwan's Medical Care Act (this site is based in Taiwan; that article describes this site's own regulatory position and is not a statement about the rules where the reader lives). It is not a medical advertisement and does not recommend any particular clinic. Wisdom tooth removal, caries restoration and replacement of missing teeth carry risks and contraindications; the actual treatment method and its results vary from person to person and must be assessed by a dentist. The prevalences, proportions and intervals cited in this card are group-level research figures. They cannot be used to estimate an individual's outcome and cannot replace a clinical diagnosis. This card makes no claim that any tooth ranks above another in importance.

Every clinical statement in this article is mapped line by line to its cited source (see the sources and evidence chain below). It has not been clinically reviewed by a licensed practitioner. This is health information, not individual advice; assessment by a clinician is required.