# Which tooth is number 21?

Answer: In the Universal Numbering System, tooth 21 is the lower left first premolar (mandibular left first premolar). Its FDI (ISO 3950) designation is 34 and its Palmer designation is 4 in the lower left quadrant. It is a premolar in the lower arch with typically 1 root.

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Answer intent ID: `tooth-number-21`
Cluster: Tooth numbering reference
Last verified: 2026-07-08
Reviewer: see source page

## Direct answer

In the Universal Numbering System, tooth 21 is the lower left first premolar (mandibular left first premolar). Its FDI (ISO 3950) designation is 34 and its Palmer designation is 4 in the lower left quadrant. It is a premolar in the lower arch with typically 1 root.

## Query patterns

- Which tooth is number 21?
- Tooth 21: Lower Left First Premolar
- Which tooth is 21?
- What is tooth number 21 in FDI and Palmer notation?
- Which tooth is FDI 34?
- What is FDI 34 in Universal numbering?
- FDI 34 to Universal tooth number
- ISO 3950 tooth 34
- Which tooth is Palmer 4 Lower Left?
- Palmer 4 Lower Left to Universal
- Lower Left Palmer 4 tooth
- LL Palmer 4 permanent tooth
- Lower Left First Premolar tooth number
- Universal 21 FDI 34 Palmer 4
- which tooth is number 21?
- tooth 21
- tooth number 21
- universal tooth 21
- adult tooth 21
- lower left first premolar
- mandibular left first premolar
- fdi 34

Source citations:
- `TOOTH-1` American Dental Association — Universal Numbering System. <https://www.ada.org/publications/ada-news/2020/november/how-to-count-teeth-numbering-systems>
- `TOOTH-2` ISO 3950:2016 — Dentistry, designation system for teeth and areas of the oral cavity (FDI two-digit). <https://www.iso.org/standard/68292.html>
- `TOOTH-3` FDI World Dental Federation — two-digit notation. <https://www.fdiworlddental.org/>

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## Supporting public URLs

- [/tools/tooth-numbering-chart](https://dentovio.com/tools/tooth-numbering-chart)
- [/tools/fdi-to-universal-converter](https://dentovio.com/tools/fdi-to-universal-converter)
- [/tools/palmer-notation-converter](https://dentovio.com/tools/palmer-notation-converter)
- [/tools/tooth-numbering-chart/tooth-21](https://dentovio.com/tools/tooth-numbering-chart/tooth-21)

## Related answers

- [FDI to Universal tooth number converter](https://dentovio.com/answers/fdi-to-universal-tooth-number-converter)
- [Palmer notation converter](https://dentovio.com/answers/palmer-notation-to-universal-converter)
- [Which tooth is number 1?](https://dentovio.com/answers/tooth-number-1)
- [Which tooth is number 2?](https://dentovio.com/answers/tooth-number-2)