关键
guānjiàn · HSK 5
crux
🔑 关 (gate/close) + 键 (key) = the key to the gate — the crux
关 means gate or to close; 键 is a key (note the 钅 metal radical). Together: the key that opens the critical gate, hence the crux, the turning point, the issue that unlocks everything else.
关 — door 门 + simplified 关 inside
关 is the simplified form of 關. The traditional character showed a gate (門) with a bolt or crossbar inside, depicting a barred door — hence 'to close' or 'shut'. Meaning later extended to 'pass' (a guarded gate), 'barrier', and figuratively 'concern' or 'connection'.
键 — metal radical 钅 + 建 jiàn (phonetic)
A semantic-phonetic compound. The metal radical 钅 signals the meaning category — originally a metal pin or bolt used to fasten a cart's wheel to its axle. The right side 建 supplies the sound 'jiàn'. The meaning later extended to 'key' and, in modern usage, the keys of a keyboard or piano.