钥

Left metal radical, the side-form of 金 — the indexing radical. Anchors 钥 in the metalwork family alongside 锁 lock, 钉 nail, 钢 steel, 银 silver. The choice of radical is concrete: keys were historically iron or bronze objects.

Right 月 — the simplified contraction of the original 龠 (a panpipe/flute) which supplied the sound yuè. In the traditional 鑰, the phonetic was the full multi-stroke 龠; the 1956 reform replaced it with the visually clean 月 (a graphic stand-in, not the moon-meaning). Provides the sound yào through historical drift.