间 jiān = 门 (mén, gate) enclosing 日 (rì, sun). The picture is light showing through the crack between a pair of closed doors: the gap itself, the space that lies between. The traditional form 間 sets 月 (yuè, moon) in the gateway instead, moonlight slipping through the door at night; the simplified 间 swaps in 日, the sun. Either way the meaning is the same, the chink of light that marks an opening, an interval, a space-between.

From that gap, four senses open out:

1. Space between, interval. The literal gap: 之间 zhījiān (between), 空间 kōngjiān (space), 间隔 jiàngé (an interval).
2. A span of time. The interval read on the clock and calendar: 时间 shíjiān (time), 期间 qījiān (a period), 中间 (in the middle of a stretch).
3. A room. A bounded space within a building, and the measure word for counting such spaces: 房间 fángjiān (room), 一间屋子 yì jiān wūzi (one room).
4. Among, within a domain. The space inside a sphere of life: 人间 rénjiān (the human world), 民间 mínjiān (among the people, folk), 田间 (in the fields).

A second reading, jiàn, foregrounds the gap as separation: 间接 jiànjiē (indirect, by way of an intermediary) and 离间 líjiàn (to sow discord, to drive a wedge between). The thread through every sense is the same opening between the doors: a gap in space, in time, or in the fabric of a relationship.

A room is a bounded space within walls, so the "space-between" character naturally came to name it. 房间 fángjiān is the everyday word for a room, and 间 also serves as the measure word for counting them: 一间房 yì jiān fáng (one room), 三间教室 sān jiān jiàoshì (three classrooms), 一间小店 (a small shop). When you count rooms, shops, or other walled spaces, 间 is the word that does the counting.

The same logic builds the names of specific rooms: 车间 chējiān (a workshop, the "vehicle space" of a factory floor), 卫生间 wèishēngjiān (bathroom, the "hygiene room"), 洗手间 xǐshǒujiān (washroom). In each, 间 marks off one enclosed space from the rest of the building, the architectural cousin of the gap between the doors.

Picture a pair of closed doors, 门, with a thin blade of sunlight, 日, showing through the crack where they meet. That sliver of light is the whole character: 间 is the gap between, the space-between made visible.

Widen the gap and it becomes a span of time (时间), the interval between two moments. Set it in a row of people and it becomes the middle (中间). Wall it off and it becomes a room you can count one by one (一间房). Open it inside a relationship and it becomes either closeness with no gap (亲密无间) or the wedge that drives people apart (离间). Wherever 间 goes, look for the space between two things.