同 tóng shows a 口 (kǒu, mouth) set under an enclosing top. It is an associative compound: the outer frame is read as a cover or a single rim, and beneath it sits the mouth. The image traditionally drawn from this is of many voices speaking from under one roof, sounding as one, hence the core sense of being together and the same. Whatever the precise origin of the upper element, the meaning that settled on the glyph is unity: things brought under a common cover, voices in unison.

From that unity the character spreads into three working senses:

1. Same, alike. Sharing one nature: 相同 xiāngtóng (identical), 同样 tóngyàng (the same, likewise), and the negative 不同 bùtóng (different).
2. Together, in common. Acting as one: 一同 yītóng (together), 共同 gòngtóng (jointly), 同时 tóngshí (at the same time).
3. The shared-role prefix. Marking people who share a station: 同学 tóngxué (fellow student), 同事 tóngshì (colleague), 同志 tóngzhì (comrade, one of the same aspiration).

同 also works as a quiet preposition meaning "with" or "and," interchangeable in many sentences with 跟 gēn and 和 hé. The thread through every use is shared ground: one nature, one action, one station, one company.

When 同 means "together," it also quietly does the work of a preposition. 我同你一起去 "I'll go with you" uses 同 exactly where many speakers would use 跟 gēn or 和 hé. All three can link two parties in a shared action, but they carry different registers: 和 is the most neutral and most common in writing, 跟 is the most colloquial and northern, and 同 sits in between, slightly formal and common in set phrases like 同…合作 "to cooperate with." When you see 同 standing before a person, test whether "with" fits; it usually does.

From this "togetherness" grows a family of words for joint action: 共同 gòngtóng (in common, jointly), 一同 yītóng and 一起 yīqǐ (together), 同意 tóngyì (to agree, literally "same idea"), and 同时 tóngshí (at the same time). Each takes the shared cover of 同 and applies it to a different thing held in common: a goal, a place, an opinion, a moment.

Picture a single 口 (mouth) sheltered under one wide cover. That is the whole character: voices brought under one roof, speaking as one. Everything 同 means flows from that single image of shared ground.

When two things share one nature, they are 相同 (the same). When people share one action, they do it 一同 or 共同 (together). When they share one station, they are named by it: fellow study makes a 同学, shared work a 同事, a common cause a 同志. And when 同 leans against a person, it quietly becomes "with." Wherever the cover falls, it gathers separate things into one.