Tooth number 20 on a dental chart is the lower left second premolar, the small tooth that sits between your lower left first premolar and your lower left first molar. This numbering follows the Universal Numbering System used by dentists across the United States and in much of North American dental practice. It is a tooth that plays a quiet but important role in chewing, and it turns out to have some quirks worth knowing about, from being one of the teeth most likely to never develop at all to having hidden extra root canals that can surprise even experienced dentists.
Where Tooth 20 Sits in Your Mouth
If you run your tongue along your lower left teeth starting from the back, you will pass the wisdom tooth (if you still have it), then two molars, then arrive at tooth 20. It is the second of two premolars on that side, positioned fifth from the center of your lower jaw. Premolars are sometimes called bicuspids because they typically have two pointed cusps on top, though, as we will see, the lower second premolar can have three. Tooth 20 sits in a transitional zone between the broader, flatter molars behind it and the narrower, sharper canine and incisors in front. Its job is to help crush and grind food as you chew, bridging the tearing function of the canine with the grinding power of the molars.
How the Universal Numbering System Works
The Universal system assigns every permanent tooth a number from 1 to 32. Numbering starts at your upper right third molar, which is tooth 1, then counts forward along the upper arch to your upper left third molar at tooth 16. It then drops down to the lower left third molar, which is tooth 17, and counts forward along the lower arch back to the lower right third molar at tooth 32. Think of it as a big U shape traced from your upper right, across to upper left, down to lower left, and across to lower right.
Under this system, tooth 20 is three teeth ahead of tooth 17 (the lower left wisdom tooth), placing it squarely in the second premolar position. If a dentist says “number 20 has a cavity,” they mean one specific tooth with no ambiguity. This is the system you will encounter at virtually any dental office in the United States, and it is the one used on insurance claim forms and treatment plans filed in the U.S.
The Same Tooth in Other Numbering Systems
Outside North America, you are more likely to encounter the FDI (Fédération Dentaire Internationale) system, which is the international standard adopted by the World Health Organization. In the FDI system, every tooth gets a two-digit code. The first digit identifies the quadrant (1 for upper right, 2 for upper left, 3 for lower left, 4 for lower right), and the second digit identifies the tooth’s position counting outward from the midline. The lower left second premolar is tooth 35 in the FDI system: quadrant 3, fifth tooth from the center.
There is also the Palmer notation system, popular in the United Kingdom, which uses a grid symbol to indicate the quadrant and a simple number (1 through 8) for the tooth’s position. Under Palmer, the lower left second premolar is written as 5 with an L-shaped bracket indicating the lower left quadrant. If you travel internationally and see dental records using unfamiliar numbers, the tooth itself has not changed. It is the same lower left second premolar regardless of what label the chart gives it.
What Tooth 20 Actually Looks Like
The mandibular second premolar is one of the more variable teeth in the human mouth. Most people picture premolars as simple two-cusped teeth, but the lower second premolar frequently departs from that template. It comes in two main forms: a two-cusp version and a three-cusp version. The three-cusp form has one prominent buccal cusp (the one facing the cheek) and two smaller lingual cusps (facing the tongue). When there are two lingual cusps, the one closer to the front of the mouth tends to be larger than the one toward the back, with a surface-area ratio of roughly 3 to 2.
You can tell these forms apart by looking at the pattern of grooves on the biting surface. A Y-shaped groove pattern is linked to the three-cusp form, while H-shaped and U-shaped groove patterns go with the two-cusp form.1Journal of Neonatal Surgery. Prevalence of Morphological Variations in The Occlusal Aspect of Mandibular Second Premolar Research on these groove patterns has found that the relative dominance of each pattern varies across populations. One Korean study found the H-pattern in about 42% of cases, the Y-pattern in about 38%, and the U-pattern in about 20%.2Journal of Technologic Dentistry. Three-dimensional analysis of the cusp variation patterns of mandibular second premolar in Koreans A study of a South Indian population, by contrast, found the Y-pattern to be most common overall.1Journal of Neonatal Surgery. Prevalence of Morphological Variations in The Occlusal Aspect of Mandibular Second Premolar The variation is interesting, though it has no real impact on how the tooth functions for you day to day. What matters more is that the buccal cusp is substantially larger than the lingual side, accounting for roughly two-thirds of the crown’s surface area, which gives the tooth its characteristic asymmetric shape when viewed from above.2Journal of Technologic Dentistry. Three-dimensional analysis of the cusp variation patterns of mandibular second premolar in Koreans
One of the Teeth Most Likely to Be Missing From Birth
Some people never develop a tooth 20 at all. The condition where one or more permanent teeth fail to form is called hypodontia, and the lower second premolar is one of the teeth most frequently affected. When a tooth is congenitally absent, the baby tooth it was supposed to replace simply never gets pushed out by a permanent successor, and no adult tooth is waiting underneath.
Most people with hypodontia are missing only one or two teeth, and permanent second premolars and upper lateral incisors are the teeth most likely to be absent.3PubMed Central. Hypodontia: An Update on Its Etiology, Classification, and Clinical Management One study found that, among patients missing teeth, upper lateral incisors accounted for about 53% of cases and lower second premolars accounted for about 26%.4PubMed Central. Evaluation of Tooth Size in Patients with Congenitally-Missing Teeth If your dentist takes an X-ray during childhood or adolescence and finds no permanent tooth bud beneath the baby molar in that spot, the options generally come down to preserving the baby tooth as long as possible, closing the space orthodontically, or eventually placing an implant or bridge.
What Happens When Tooth 20 Gets Stuck
Even when the tooth does develop, it does not always make its way into the mouth on schedule. The mandibular second premolar is one of the more commonly impacted teeth after wisdom teeth, meaning it can get trapped beneath the gum line or blocked by neighboring teeth.5PubMed Central. Early Diagnosis and Treatment of Mandibular Second Premolar Impaction: A Case Report This tends to be caught in childhood or early adolescence, when routine dental X-rays reveal that the tooth is heading in the wrong direction or has stalled in its path.
The reason this matters beyond cosmetic alignment is that an impacted tooth sitting beneath the gum can cause real problems over time. Left untreated, it can lead to cyst formation around the trapped crown, risk of infection, and damage to the roots of teeth next to it.6Orthodontic Update. When Doing Nothing is an Option: Self-correction of an Impacted Mandibular Second Premolar In some cases, an impacted second premolar will eventually self-correct and erupt on its own, especially if the baby tooth above it is lost or extracted to make room. But early detection through X-rays gives the dentist more options, including guided eruption with braces or surgical exposure, than a late discovery does.
Root Canal Complications Specific to Tooth 20
If you ever need a root canal on tooth 20, the anatomy inside the tooth can throw a curveball. Mandibular second premolars are widely taught as single-canal teeth, and that is true in the large majority of cases. But the tooth has a bit of a reputation in endodontics for harboring hidden extra canals. Case reports describe mandibular second premolars with two canals and, more rarely, with three completely independent roots and canals.7PubMed Central. Root canal treatment of a mandibular second premolar with three roots and canals – an anatomic variation These extra canals are genuinely rare, but when they are present and go undetected, the root canal treatment can fail because bacteria survive in the untreated canal.
The clinical challenge is that these accessory canals are not always visible on a standard two-dimensional X-ray. They can be hidden behind the main canal or merge and split at odd levels within the root.8PubMed Central. The Hidden Anatomy of a Lower Premolar With Two Canals Cone-beam CT scans, which produce three-dimensional images, are increasingly used when a dentist suspects unusual root anatomy. If you have had a root canal on this tooth that did not resolve your symptoms, one possible explanation is a missed canal. This is not a reason to panic, since most second premolars do have a straightforward single canal, but it is worth knowing about if you are dealing with persistent pain after treatment.
Why Losing the Baby Tooth Early Matters Here
Tooth 20 replaces the deciduous (baby) second molar, which sits in its spot until the permanent premolar is ready to erupt, usually around age 11 or 12. If that baby molar is lost early due to decay, trauma, or extraction, the neighboring teeth can drift into the empty space. The first molar behind it tends to tip forward, and the teeth in front can shift backward, leaving insufficient room for the permanent second premolar to come in straight.
This is why pediatric dentists pay close attention to premature loss of baby second molars. Space maintainers, small metal or acrylic devices cemented to the adjacent teeth, can hold the gap open until the permanent tooth is ready. Research supports the idea that space maintainers preserve arch length, keeping the surrounding teeth from crowding into the gap.9European Journal of Orthodontics. Effect of treatment with dental space maintainers after the early extraction of the second primary molar: a systematic review However, there are trade-offs. Fixed space maintainers tend to increase plaque buildup around the device, making oral hygiene more important during the years the maintainer is in place. Without a space maintainer, the lower first molar behind the gap can tilt significantly toward the empty space, which can create bite problems that require orthodontic correction later.9European Journal of Orthodontics. Effect of treatment with dental space maintainers after the early extraction of the second primary molar: a systematic review The general principle is that keeping the baby tooth in place as long as possible is the ideal scenario, since the natural tooth is the best space maintainer there is.10PubMed Central. Orthodontic Space Management in Pediatric Dentistry: A Clinical Review
Common Confusions With Dental Numbers
A frequent source of confusion is that different numbering systems assign different numbers to the same tooth. If you have dental records from another country, a tooth labeled 35 in the FDI system is the same tooth as number 20 in the Universal system. Mixing the two up can cause genuine problems if records are transferred between countries without conversion. Forensic odontologists, who use dental records to identify individuals, have to be especially careful about which system was used in the original charting.
Another common mix-up happens between upper and lower premolars. Tooth 20 (lower left second premolar) and tooth 13 (upper left second premolar) occupy roughly mirroring positions in the mouth, but they are different teeth with different anatomy. The upper second premolar tends to have two cusps of more equal size, while the lower one, as described above, has a dominant buccal cusp and more variable lingual anatomy. If someone tells you “you have a problem with premolar number 20,” it is always the lower left one.
Perhaps the most practical piece of advice is simply to ask your dentist to point to the tooth on a chart or in your mouth if you are confused by a number. Dental professionals are used to the numbering and sometimes forget that patients are not. No one will think less of you for asking “which one is that?” when a treatment plan references a number you cannot picture.
When Tooth 20 Is the Extraction Choice in Orthodontics
In orthodontic treatment for crowding, premolars are the teeth most commonly extracted to create space. The decision between removing first premolars or second premolars depends on several factors, including the severity of the crowding, the position of the canines, and the patient’s facial profile. Second premolars are sometimes preferred for extraction when the crowding is moderate and the orthodontist wants to keep the first premolars, which sit closer to the canines and help maintain the shape of the front part of the arch.
If tooth 20 is congenitally absent, an orthodontist may actually use that naturally missing tooth to their advantage. The absent tooth means there is already potential for space consolidation on that side. Treatment planning in these cases often involves closing the gap by moving the first molar forward, which avoids the need for an implant or prosthetic replacement entirely. The flip side is that this only works when the bite and crowding pattern cooperate. If the gap needs to stay open for a balanced bite, the orthodontist will plan for a future implant or bridge instead.
Tooth 20 might not be the most glamorous member of your dental lineup, but it occupies an interesting intersection of dental anatomy, development, and treatment planning. Whether it erupts normally, never forms, or hides an extra root canal, it gives dentists plenty to think about for a tooth most people barely notice.