Tooth Number 21: Lower Left First Premolar

Root canal on tooth 21

What it involves. Root canal treatment removes inflamed or infected pulp from inside the tooth, then cleans, disinfects, fills and seals it. It can usually be done in one or two visits, and the tooth is then restored with a crown or filling.

One root, sometimes two canals. A review of over 6,700 lower first premolars found one root in about 98%, but about 1 in 4 (24.2%) had two or more canals. Extra canals and roots were reported more often in Chinese, Australian and sub-Saharan African populations.

Because of that, dentists look carefully for a second canal before root canal treatment.

The same tooth on the right side, tooth 28, has its own section on removal for braces and the nerve nearby.

Where is tooth 21?

Tooth 21 is in your lower jaw on your left side. It is the fourth tooth from the middle: start at the gap between the two bottom front teeth and count 4 teeth toward the back on your left side.

In a mirror, tooth 21 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 22, the lower left canine. Behind it is tooth 20, the lower left second premolar.

Its mirror twin on the right side is tooth 28. The tooth in the same spot in the upper jaw is tooth 12, the upper left first premolar.

Dental charts show the patient's right side on the left, so tooth 21 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 first 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 first premolar has a large, pointed cheek-side cusp and a small tongue-side cusp, so it looks a bit like a canine.

It is also called the "first bicuspid".

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

How many roots does tooth 21 have?

Almost always 1 root.

When does tooth 21 come in?

Both the ADA and AAPD charts give 10–12 years for the lower first premolars.

Tooth 21 replaces the lower left baby first molar (tooth L), which usually falls out at 9–11 years (ADA).

In the ADA chart, they come in at the same time as the upper second premolars, after the upper first premolars (10–11 years) and before the upper canines (11–12 years). In the AAPD's 13-step order, from the first molars (step 1) to the wisdom teeth (step 13), they are step 8.

Under the gum, according to the AAPD chart, it starts to harden (calcify) at about 18–24 months, has a finished crown by 5–6 years and has a finished root by about 12–13 years, a few years after it comes into the mouth.

Common problems with lower first 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 21 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 21 in FDI and Palmer notation

"21" in FDI is a different tooth. Read as an FDI code, 21 means the upper left central incisor (tooth 9 in Universal). Charts and X-ray reports from outside the US often use FDI, so check which system is in use.

"34" can only be FDI. Universal numbers stop at 32, so a tooth 34 on any chart is this tooth.

Palmer needs the bracket. Every quarter of the mouth has a tooth 4 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 21

• 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. World Health Organization. Oral health surveys: basic methods, 5th edition (tooth notation after ISO 3950). 2013. Accessed October 26, 2026.
5. Nelson SJ. Elsevier. Wheeler's Dental Anatomy, Physiology and Occlusion, 11th edition. 2019.
6. American Dental Association. ADA Dental Claim Form (tooth designation systems). Accessed October 26, 2026.
7. Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Types of Teeth and their Functions. Accessed October 26, 2026.
8. Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Permanent Dentition. Accessed October 26, 2026.
9. Cleveland Clinic. Teeth: Anatomy, Types, Function & Care. last updated January 26, 2023. Accessed October 26, 2026.
10. American Association of Endodontists. What is a Root Canal?. Accessed October 26, 2026.
11. Cleghorn BM, Christie WH, Dong CC. Journal of Endodontics 33(5):509–516. The root and root canal morphology of the human mandibular first premolar: a literature review. 2007. Accessed October 26, 2026.

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