合 hé pictures a lid coming down over the mouth of a vessel. The top element is 亼 (jí), an inverted, roof-like shape read as a cover, and below it sits 口 (kǒu), the round opening of a container drawn as a mouth. Set one over the other and you see a lid settling onto a jar so that the two edges meet and seal. The early sense is exactly that: two parts brought together so they fit and close. The simplified and traditional forms are identical; there is no separate traditional glyph.

From that one image of edges meeting, three related senses spread across thousands of words:

1. To join, to combine. Two things brought into one: 合作 hézuò (to cooperate), 结合 jiéhé (to combine; to integrate), 联合 liánhé (to unite), 合并 hébìng (to merge).
2. To suit, to match, to fit. When the lid sits true on the jar, the parts agree: 合适 héshì (suitable), 适合 shìhé (to suit), 符合 fúhé (to conform to), 合理 hélǐ (reasonable).
3. To close; the whole. The closing motion itself, and the sum it produces: 合上 héshàng (to close up), 合眼 héyǎn (to close the eyes), 合计 héjì (to total up).

The thread running through every sense is two edges meeting cleanly. Whether the parts are people (cooperation), measurements (a good fit), an argument (reasonable), or eyelids (shutting), 合 is always the moment when separate things come together and agree.

The most literal sense of 合 is the physical act of closing: bringing two edges together until they meet. 合上 héshàng means to close something that opens along a seam, a book or a lid or a door: 合上书 "to close the book." 合眼 héyǎn is to close the eyes, often a gentle way of saying to fall asleep or, in mourning, to pass away peacefully.

From closing comes merging, and from merging comes the idea of a whole made of parts. 合并 hébìng joins two separate units into one: two companies, two departments, two cells. 综合 zōnghé gathers many strands into a single comprehensive view. Even 合唱 héchàng, a chorus, is voices closed together into one sound. Across all of these, 合 carries the same motion the etymology shows: parts drawn in until they form a single, sealed whole.

See a lid, 亼, settling down onto the mouth of a jar, 口, until the two rims meet and seal. That single closing motion is the whole character. Everything 合 means is parts coming together until they fit.

When the parts are people, the meeting is cooperation (合作). When the parts are a thing and its setting, it is a good fit (合适). When the parts are a claim and a standard, it is conformity (符合). When the parts are two units, it is a merger (合并). And when the parts are simply two edges of a book, it is the plain act of closing (合上). Whenever you meet 合, ask what two things are being brought together, and you will have its sense.