# Tooth O: Primary Lower Left Central Incisor

> In the Universal Numbering System, primary tooth O is the primary lower left central incisor (primary mandibular left central incisor). Its FDI (ISO 3950) designation is 71 and its Palmer designation is A in the lower left quadrant. It is a incisor in the lower arch with typically 1 root.

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Last verified: 2026-07-08

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

## Direct answer

- Which baby tooth is letter O? In the Universal Numbering System, primary tooth O is the primary lower left central incisor (primary mandibular left central incisor). Its FDI (ISO 3950) designation is 71 and its Palmer designation is A in the lower left quadrant. It is a incisor in the lower arch with typically 1 root.
- Universal letter: O
- FDI / ISO 3950 designation: 71
- Palmer designation: A LL
- Plain-language name: Primary Lower Left Central Incisor
- Anatomical name: Primary Mandibular Left Central Incisor
- Quadrant: Lower Left
- Arch and side: lower, left
- Type: incisor
- Typical roots: 1

## Conversion table

| System | Designation |
| --- | --- |
| Universal Numbering System | O |
| FDI / ISO 3950 | 71 |
| Palmer notation | A LL |
| Tooth name | Primary Lower Left Central Incisor |

## Search terms

- which baby tooth is letter o?
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- lower left tooth 1
- lower left incisor

## 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 N: Primary Lower Left Lateral Incisor](https://dentovio.com/tools/tooth-numbering-chart/tooth-n/index.html.md)
- [Next tooth: Tooth P: Primary Lower Right Central Incisor](https://dentovio.com/tools/tooth-numbering-chart/tooth-p/index.html.md)