Pictograph: a pair of double doors hanging side by side. Oracle bone forms show two flat door panels with a slight gap between — exactly the image of a traditional wooden gate. Three forms: 繁體 門 / 新字体 門 / 简体 门 (the simplified form keeps the shape but reduces the strokes).

門 / 门 is one of the most productive radicals in the script. When you see 門 enclosing other components, predict "door / opening / category": 開 / 开 (open), 閉 / 闭 (close), 間 / 间 (between / interval), 問 / 问 (ask), 聞 / 闻 (hear / smell), 関 / 关 (gate / barrier), 閥 (clique). Spotting the 門 frame around any character cuts the meaning space dramatically.

The character's meaning extended from physical doors to abstract "category / branch / school": 専門 / 専門 (specialty), 部門 (department), 入門 (introduction / "entering the gate" of a discipline).

Mandarin: mén, rising 2nd tone (simplified 门). 门 (mén, door), 开门 (kāimén, open the door), 关门 (guānmén, close the door), 门口 (ménkǒu, doorway / entrance), 部门 (bùmén, department), 大门 (dàmén, main gate), 门票 (ménpiào, entrance ticket).

Japanese: on-reading モン (mon) is dominant — 専門 (senmon, specialty / major), 入門 (nyūmon, introduction / "entering the gate" — used for textbook titles like 日本語入門), 大門 (daimon, large gate), 部門 (bumon, department), 校門 (kōmon, school gate), 門限 (mongen, curfew). Kun-reading かど (kado) appears in 門出 (kadode, the start of a journey or new chapter in life — culturally weighty term used at graduations and weddings).

The Japanese 鳥居 (torii — the iconic vermilion gates marking Shinto shrines) functions as a 門 — a sacred threshold between worldly and sacred space.

Memory aid: two door panels — the picture is the meaning.