Several parts combine into one character.

婿 is a phonetic-semantic compound: 女 (woman) + 胥 (xū, "mutual / each other" — providing the sound). The composite reads as "the man partnered with the daughter / the daughter's husband" = son-in-law. 婿 forms the gendered counterpart to 嫁 (bride / marrying-out): 嫁 emphasizes the woman's movement to a new household, 婿 emphasizes the man becoming his in-laws' family member. Together they map the traditional marriage transaction from both directions.

Korean reading "seo." Korean preserves 婿 only in formal classical compounds: 女婿 (yeoseo, son-in-law — formal), 快婿 (kwaeseo, "fine son-in-law" — used in matchmaking literature), 小婿 (soseo, humble first-person used by a son-in-law referring to himself in formal letters to his wife's parents). Modern Korean overwhelmingly uses the native (sawi) for "son-in-law" in everyday speech.

Mandarin xù, 4th tone. 女婿 (nǚxù, "daughter's husband" = son-in-law — the everyday word), and the auspicious idiom 乘龙快婿 (chénglóng kuàixù, "an excellent son-in-law riding a dragon" — referring to a stellar marriage match the bride's family is delighted with). Chinese traditional marriage culture invests considerable language in describing the qualities of the ideal son-in-law.

Japanese on-reading セイ (sei) is rarely used. The kun-reading むこ (muko) is the everyday word: 婿 (muko, son-in-law / bridegroom), 花婿 (hanamuko, "flower bridegroom" = bridegroom — used in formal wedding contexts), 婿養子 (muko-yōshi, "son-in-law adoption" — a uniquely Japanese practice where the husband formally enters his wife's family as an adopted son, taking her surname; used historically by daughters of families without sons to preserve the family name and assets). 花嫁 (hanayome, bride) and 花婿 (hanamuko, groom) appear together on every Japanese wedding invitation.

Memory aid: woman (女) + 胥 (mutual / each other) — the man matched to the daughter = son-in-law.