必要
bìyào · HSK 3
necessary
✅ 必 = certain/must + 要 = want/need → must-want = necessary
必 carries the sense of must or certainly; 要 means to want or to need. Together they form must-need, i.e. something indispensable. Compare 必须 (must) and 需要 (need) — 必要 sits between them as the noun/adjective necessary.
必 — heart 心 + slash 丿
Originally depicted a handle or marker driven through something, suggesting a fixed standard. The form was later reanalyzed as 心 (heart) with a stroke through it, reinforcing the sense of inner certainty. It came to mean 'must' or 'certainly'.
要 — west 覀 (top) + woman 女 (bottom)
Originally a pictograph of a woman with hands on her waist, meaning 'waist' (the original form of 腰). The top 覀 is a stylized depiction of the waist area, not the character for 'west'. Over time it was borrowed for the meaning 'to want, important', so 腰 (with 月 flesh radical) was created for the original 'waist' sense.