本 is one of the most elegant 指事字 (indicator characters) in the entire CJK system. Take 木 (tree) and add a single horizontal stroke at the base — that stroke "points to" the root, the foundation, the origin of the tree. The opposite character 末 (mò, "end / tip") works the same way, but adds the stroke at the top of 木 to point to the branches and tip. One radical, two opposite indicators differing by where a single stroke is placed: bottom = root / origin, top = end / extremity. The pair 本末 (běn-mò) idiomatically captures "root and tip" / "origin and end" / "essential and trivial."

Korean reading "bon." 本來 (bollae, originally / fundamentally), 根本 (geunbon, fundamental root), 基本 (gibon, basics / fundamentals), 本人 (bonin, oneself — used in formal address: "본인은..." = "I myself..."), 本店 (bonjeom, head store / main shop), and the canonical 日本 (Ilbon, Japan — the country whose name means "origin of the sun"). Korean preserves the "root / origin" semantic core most strictly across registers.

Mandarin běn, 3rd tone. 本 (běn), 本来 (běnlái, originally), 基本 (jīběn, basic), 本人 (běnrén, the person himself), 课本 (kèběn, textbook). And critically: 本 functions as a measure word for books — 一本书 (yī běn shū, "one volume book"). Every book in Mandarin is counted with 本. This grammaticalization of 本 as a noun classifier is unique to Mandarin among the CJK languages.

Japanese is where 本 became most semantically prolific. On-reading ホン (hon) — 本 itself read as ほん means "book" (this is the everyday Japanese word for book), 日本 (Nihon / Nippon, Japan), 基本 (kihon, basics), 本気 (honki, "true spirit" = serious / genuine), 本当 (hontō, "true" = really / truly — extremely common in spoken Japanese), 本物 (honmono, the real thing). Kun-reading もと (moto, "origin / source") survives in compounds: 根本 (nemoto, "root-base" = roots), 元 (also moto, with different kanji). The original "root" meaning fanned out into five distinct semantic clusters in Japanese: book, origin, original/genuine, true, this/that one (counter for cylindrical objects).

In Japanese the character also serves as a counter for long thin objects: 鉛筆一本 (enpitsu ippon, "one pencil"), パン三本 (pan sanbon, "three loaves of bread") — paralleling but distinct from the Mandarin book-counter usage.

Memory aid: a tree (木) with a stroke pointing to its base — the root, origin, foundation. The opposite, 末, points to the tip with a stroke at the top.