Several parts combine into one character.

伯 is a phonetic-semantic compound: 亻 (person) + 白 (bái, "white" — providing the sound). The composite suggests "a person with white hair / an elder, the head" — and from this image of the senior figure grew the meaning "eldest brother, eldest of the four ranks." 伯 holds the first position in the classical sibling-hierarchy system 伯·仲·叔·季 (eldest, second-born, third-born, youngest). From the family-rank sense came two further extensions: (1) "father's elder brother" — paternal uncle older than father; (2) "count / earl" — the European nobility rank when European feudal titles were translated into East Asian languages, 伯爵 was assigned to the rank of count/earl in CJK historical translations.

Korean reading "baek." 伯父 (baekbu, father's elder brother — formal), 伯母 (baengmo, the wife of father's elder brother), 伯爵 (baekjak, count / earl — used in translations of European fiction and history), 伯仲 (baekjung, "eldest and second-born" = "evenly matched / nip and tuck" — a four-character idiom 백중지간 baekjung-jigan describing competitors of comparable strength). The 백중 idiom is one of the most evocative Korean Sino-Korean compounds.

Mandarin bó, 2nd tone. 伯伯 (bóbo, "uncle-uncle" = paternal uncle older than father — used affectionately by children), 伯父 (bófù, formal paternal uncle older than father), 伯母 (bómǔ, paternal aunt by marriage to father's elder brother), 伯爵 (bójué, count). Mandarin distinguishes 伯 (paternal uncle older than father) from 叔 (paternal uncle younger than father) precisely; the kinship terminology grid is more granular than English "uncle."

Japanese on-reading ハク (haku) — 伯父 (haku-fu, but typically read as お じ "oji" via jukujikun), 伯母 (haku-bo, read as お ば "oba"), 伯爵 (hakushaku, count). Japan replicates the same kinship-rank distinction as Mandarin: 伯父 / 伯母 = parent's elder sibling; 叔父 / 叔母 = parent's younger sibling. Both pairs are pronounced identically as oji/oba in spoken Japanese; the kanji choice is mandatory in writing and reveals attention to the parent's sibling order.

Memory aid: the four sibling positions: 伯 (eldest), 仲 (second), 叔 (third), 季 (youngest). 伯 = the eldest, the white-haired senior.