Compound ideograph: 止 (foot/footprint) on top + 儿 (a person, the legs-radical form) below. The picture: a foot striding ahead while the rest of the person follows. Encoded meaning: "the one who went first." Identical across 繁體 / 新字体 / 简体.

The key compound across CJK is 先生 ("the one born first / the elder") — a single word that adapted differently in each language: — Mandarin 先生 (xiānsheng): now means "Mister" or "Sir". 王先生 (Wáng xiānsheng) = "Mr. Wang". Used as a polite address, no longer specifically meaning teacher. — Japanese 先生 (sensei): still means "teacher / doctor / professor / master" — the classical respect-for-elder sense survived. Used for school teachers, doctors, lawyers, and martial arts masters. NEVER use sensei for yourself. — Korean (seonsaeng): also "teacher" — same as Japanese. This split — Mandarin generalizing to "Mister", Japanese/Korean preserving "teacher" — is one of the most useful CJK comparative facts to know.

Mandarin: xiān, level 1st tone. 先 (xiān, first), 首先 (shǒuxiān, first of all), 先后 (xiānhòu, before-and-after / order), 优先 (yōuxiān, priority), 先进 (xiānjìn, advanced — "first-advancing"), 祖先 (zǔxiān, ancestor — "elder first").

Japanese: on-reading セン (sen) is dominant — 先生 (sensei, teacher), 先週 (senshū, last week), 先月 (sengetsu, last month), 先輩 (senpai, senior — at school or work), 先頭 (sentō, head/lead). Kun-readings: さき (saki, ahead / tip / point) — 先に (saki ni, first / earlier / ahead), 行き先 (ikisaki, destination — "going-ahead"); まず (mazu, "first of all", a sentence-starting adverb).

The school/work hierarchy concept of 先輩 / 后辈 (senior / junior) — senpai-kōhai in Japanese — anchors classroom and office relationships in Japan, Korea, and traditional Chinese contexts. Knowing 先 is essential for understanding social structure.

Memory aid: a footprint on top, person below — the one who walked ahead.