横
よこ · yoko · JLPT N5 · noun
side
↔️ 横 = 木 tree + 黄 yellow (sound part): a wooden crossbar lying sideways
The 木 radical marks wood; 黄 only lends its sound (ō, as in 横断 ōdan, crossing). The original sense was a crossbar set across a gate, so the word came to mean sideways, width, and the space beside something. Yoko is the native reading.
横 — wood radical 木 + 黄 (huáng, phonetic)
A semantic-phonetic compound: 木 'wood' gives the meaning (a horizontal wooden bar across a gate), while 黄 supplies the sound. It originally meant the crossbar of a gate, then extended to 'horizontal' generally. The reading hèng later developed the sense of 'unruly' or 'unexpected'.