相辅相成
xiāng fǔ xiāng chéng · HSK 7
to complement one another (idiom)
🤝 相辅相成 = mutually (相) + assist (辅) + mutually (相) + accomplish (成): two parts help each other succeed
相 means each-other and repeats in slots 1 and 3, bracketing 辅 (assist) and 成 (accomplish/complete). The idiom names two things that each supply what the other lacks, so together they finish the job neither could alone.
成 — 戊 (halberd/weapon) + 丁 (nail/man)
Originally a pictograph of a halberd or axe-like weapon, suggesting the act of completing a task by force. Over time it shifted from 'to subdue' to the broader meaning 'to accomplish' or 'to become'. The 丁 element inside also hints at the chéng sound.