同士 (どうし)

• Reading: どうし
• Romaji: doushi
• Formation: Noun + 同士 (+ で / の / が)
• Kanji breakdown: 同 (same) + 士 (person, warrior)

Meaning

A suffix meaning 'fellow,' 'among the same kind,' or 'with each other,' indicating that the action or relationship involves people or things of the same category.

同士 attaches to nouns to express a mutual or reciprocal relationship between entities of the same type. 友達同士 means 'among friends' or 'between friends,' 国同士 means 'between nations,' and 似た者同士 means 'birds of a feather.' It implies that the entities share a common attribute and that the action or state described involves them interacting with or relating to each other. Unlike お互い, which explicitly means 'each other' and functions as a pronoun, 同士 is a suffix that categorizes the participants. It can be followed by で (indicating the group context), の (modifying a noun), or directly by a predicate. It is used across all registers and is especially common when describing interpersonal or inter-group dynamics.

Examples

1. 学生同士で話し合って解決策を見つけてほしい。 I want the students to discuss among themselves and find a solution.
2. 隣国同士の関係は歴史的に複雑だ。 Relations between neighboring countries are historically complex.
3. 同僚同士の信頼関係がチームの生産性を左右する。 The trust between colleagues affects the team's productivity.

Usage Guide

Context: spoken, written, everyday

Tone: descriptive

Do Say

• 専門家同士の議論は非常に高度な内容に及んだ。
• 兄弟同士でも性格が全く違うことがある。
• 企業同士の競争が技術革新を促進している。

Don't Say

• 同士の友達が遊んでいる。(Placing 同士 before the noun instead of after it) → 友達同士で遊んでいる。
• 先生と学生同士で話した。(Using 同士 for entities of different categories — it requires the same type) → 先生と学生で話した。

Origin & History

同士 is a Sino-Japanese compound from 同 (same) and 士 (person/warrior). Originally referring to comrades or people of the same rank, it broadened to a general suffix indicating shared category membership.

Cultural Context

Generation: All ages

Social background: Universal