Several parts combine into one character.

Compound ideograph: 宀 (roof) + 至 (arrive). The encoded scene: "the place under a roof where one arrives" = a room. Where 屋 (previous entry) means the whole building, 室 specifically denotes a room INSIDE the building — a single chamber within. Identical across 繁體 / 新字体 / 简体.

室 anchors institutional and architectural room-vocabulary across CJK languages: classrooms, offices, bedrooms, meeting rooms — all use this character as the room-suffix.

Mandarin: shì, falling 4th tone. 教室 (jiàoshì, classroom), 办公室 (bàngōngshì, office), 卧室 (wòshì, bedroom — "lying room"), 浴室 (yùshì, bathroom), 会议室 (huìyìshì, meeting room), 实验室 (shíyànshì, laboratory). The 〜室 suffix is one of the most productive in Mandarin technical/institutional vocabulary.

Japanese: on-reading シツ (shitsu) for compounds — 教室 (kyōshitsu, classroom), 会議室 (kaigishitsu, meeting room), 浴室 (yokushitsu, bathroom), 寝室 (shinshitsu, bedroom), 図書室 (toshoshitsu, library room — distinct from 図書館 toshokan, library building), 理科室 (rikashitsu, science classroom), 美術室 (bijutsushitsu, art classroom). Every Japanese school deploys 〜室 names extensively.

Kun-reading むろ (muro) is largely archaic in modern usage. The everyday Japanese word for "room" is the irregular reading 部屋 (heya, from previous batch entry) — NOT 室. So: — 室 (shitsu): formal/institutional rooms (classroom, office, etc.). — 部屋 (heya): casual room (my room, hotel room, living room). The register split is essential — Japanese learners must know when to use which word.

Memory aid: roof + arrival point = the room one settles into.