Tooth Number 20: Lower Left Second Premolar

Missing tooth 20

Leaving out wisdom teeth, the lower second premolar is the adult tooth most often missing from birth. Overall, 4.4% to 13.4% of people are born without at least one adult tooth, depending on where they live.

A clue in the baby molar. When this premolar never forms, the baby second molar above it (tooth K) often stays. A baby molar that seems to sink below its neighbors (ankylosis) can be a sign the premolar is missing.

The same tooth on the right side, tooth 29, has its own section on options when it never forms.

Where is tooth 20?

Tooth 20 is in your lower jaw on your left side. It is the fifth tooth from the middle: start at the gap between the two bottom front teeth and count 5 teeth toward the back on your left side.

In a mirror, tooth 20 shows on the left side of your reflection. Someone looking into your mouth sees it on their right.

In front of it (toward the middle) is tooth 21, the lower left first premolar. Behind it is tooth 19, the lower left first molar.

Its mirror twin on the right side is tooth 29. The tooth in the same spot in the upper jaw is tooth 13, the upper left second premolar.

Dental charts show the patient's right side on the left, so tooth 20 sits on the right half of the bottom row. The numbers then drop to the lower jaw: lower left teeth run from 17 (the wisdom tooth) to 24 (the front tooth).

What does the lower second premolar do?

Premolars (also called bicuspids) sit between the canines and the molars and help crush and grind food. They have features of both their neighbors. The lower second premolar is larger than the first premolar and often has two tongue-side cusps, so it can look like a small molar.

It is also called the "second bicuspid".

Adult premolars replace the baby molars. There are no premolars among the 20 baby teeth.

How many roots does tooth 20 have?

1 root.

When does tooth 20 come in?

The lower second premolars usually come in at 11–12 years according to the ADA chart. The AAPD chart gives 11–13 years.

Tooth 20 replaces the lower left baby second molar (tooth K), which usually falls out at 10–12 years (ADA).

In the ADA chart, they come in at the same time as the upper canines, after the lower first premolars (10–12 years) and before the lower second molars (11–13 years). In the AAPD's 13-step order, from the first molars (step 1) to the wisdom teeth (step 13), they are step 10.

Under the gum, according to the AAPD chart, it starts to harden (calcify) at about 24–30 months, has a finished crown by 6–7 years and has a finished root by about 12–14 years, a few years after it comes into the mouth.

Common problems with lower second premolars

• Cavities. Small cavities are fixed with a filling. If decay or an injury reaches the nerve, root canal treatment can save the tooth, often followed by a crown.

If tooth 20 hurts, feels loose, or the gum around it swells, the Dental Emergency Checker can help you decide how soon to see a dentist. Swelling that spreads to the eye or neck, or makes it hard to breathe or swallow, is an emergency: call 911. These are typical problems for this kind of tooth, not a diagnosis.

Tooth 20 in FDI and Palmer notation

There is no FDI tooth 20. In FDI the second digit only runs from 1 to 8, so 20 can only be a Universal number.

"35" can only be FDI. Universal numbers stop at 32, so a tooth 35 on any chart is this tooth.

Palmer needs the bracket. Every quarter of the mouth has a tooth 5 in Palmer notation; only the ┌ (or the letters LL) makes it this one.

All adult teeth in the lower left quarter

From the front of the mouth to the back:

The Tooth Number Chart & Converter converts any tooth between the three systems.

Tools for tooth 20

• Dental Emergency Checker: how soon to see a dentist if it hurts or breaks.
• Dental Numbness Calculator: how long numbness lasts after a filling.
• Dental Insurance Calculator: what your plan may pay for a crown or root canal.
• Adult Teeth Predictor: likely dates for your child's adult teeth.
• How Long Does Dental Work Last?: how long fillings, crowns, veneers and implants usually last, from long-term studies.
• Dental Treatment Plan Decoder: plain English for treatment-plan shorthand like "#30 MOD comp".

Frequently asked questions

Sources

1. American Dental Association (MouthHealthy). Permanent Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
2. American Dental Association (MouthHealthy). Primary Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
3. American Academy of Pediatric Dentistry. Dental Growth and Development (chronology after Logan & Kronfeld 1933, adapted 2003). Accessed October 26, 2026.
4. American Academy of Pediatric Dentistry. Management of the Developing Dentition and Occlusion in Pediatric Dentistry (Reference Manual of Pediatric Dentistry). latest revision 2024. Accessed October 26, 2026.
5. World Health Organization. Oral health surveys: basic methods, 5th edition (tooth notation after ISO 3950). 2013. Accessed October 26, 2026.
6. Nelson SJ. Elsevier. Wheeler's Dental Anatomy, Physiology and Occlusion, 11th edition. 2019.
7. American Dental Association. ADA Dental Claim Form (tooth designation systems). Accessed October 26, 2026.
8. Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Types of Teeth and their Functions. Accessed October 26, 2026.
9. Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Permanent Dentition. Accessed October 26, 2026.
10. Cleveland Clinic. Teeth: Anatomy, Types, Function & Care. last updated January 26, 2023. Accessed October 26, 2026.

Last reviewed October 26, 2026 by the Dental Health Time editorial team, against the sources above. How we review.