# Tooth 21: Lower Left First Premolar

> 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.

URL: https://dentovio.com/tools/tooth-numbering-chart/tooth-21

Markdown: https://dentovio.com/tools/tooth-numbering-chart/tooth-21/index.html.md

Last verified: 2026-07-08

Neutral tooth-designation reference only; not clinical, diagnostic, or treatment advice. Root counts describe typical permanent-tooth morphology and vary between individuals.

## Direct answer

- Which tooth is number 21? 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.
- Universal number: 21
- FDI / ISO 3950 designation: 34
- Palmer designation: 4 LL
- Plain-language name: Lower Left First Premolar
- Anatomical name: Mandibular Left First Premolar
- Quadrant: Lower Left
- Arch and side: lower, left
- Type: premolar
- Typical roots: 1

## Conversion table

| System | Designation |
| --- | --- |
| Universal Numbering System | 21 |
| FDI / ISO 3950 | 34 |
| Palmer notation | 4 LL |
| Tooth name | Lower Left First Premolar |

## Search terms

- which tooth is number 21?
- tooth 21
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- adult tooth 21
- lower left first premolar
- mandibular left first premolar
- fdi 34
- palmer 4
- lower left tooth 4
- lower left premolar

## Sources

- `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/>

## Related

- [Full tooth numbering chart](https://dentovio.com/tools/tooth-numbering-chart/index.html.md)
- [FDI to Universal converter](https://dentovio.com/tools/fdi-to-universal-converter/index.html.md)
- [Palmer notation converter](https://dentovio.com/tools/palmer-notation-converter/index.html.md)
- [Previous tooth: Tooth 20: Lower Left Second Premolar](https://dentovio.com/tools/tooth-numbering-chart/tooth-20/index.html.md)
- [Next tooth: Tooth 22: Lower Left Canine](https://dentovio.com/tools/tooth-numbering-chart/tooth-22/index.html.md)