门 (mén) — door; gate; (classifier) for courses, subjects, branches of technology, etc.

门 is the everyday word for 'door/gate' and also a classifier for courses, subjects, or fields of study. Compare 户 (households) and 派别 (sects/schools) — use 门 for courses, 户 for families, 派别 for ideological factions.

A drawing of the thing itself, worn down over three thousand years into a few strokes.

This origin belongs to the traditional form 門; 门 is its simplified writing.

The traditional form 門 pictures a gate as two door shapes side by side; the simplified 门 is a later shorthand.