Several parts combine into one character.

Phonetic-semantic compound: 穴 (cave / hole) + 工 (a phonetic tool component). The encoded meaning: "an empty space dug out / a hollow". From physical hollow, the meaning extended outward to "the great hollow above us" = the sky. So 空 sustains TWO seemingly contradictory meanings — "empty / void" AND "sky" — both unified by the underlying concept of "negative space". Identical across 繁體 / 新字体 / 简体.

This dual-meaning is essential for reading: 空 means EITHER "empty" OR "sky" depending on compound context. 空気 (air — "empty energy"), 空港 (airport — "empty harbor"), 空間 (space), 真空 (vacuum), 天空 (sky/heavens). The metaphysical Buddhist concept 空 (Sanskrit śūnyatā, "emptiness") — central to Heart Sutra philosophy — uses this character precisely.

Mandarin has TWO tones for 空: — kōng (1st tone): empty / sky / hollow. 空气 (kōngqì, air), 空间 (kōngjiān, space), 天空 (tiānkōng, sky), 空虚 (kōngxū, hollow / empty), 空白 (kòngbái — wait, this shifts to 4th). — kòng (4th tone): free time / leisure / blank. 有空 (yǒukòng, "to have time / be free"), 空闲 (kòngxián, free time), 空白 (kòngbái, blank space). The tone distinguishes "physical emptiness" (kōng) from "schedule emptiness" (kòng).

Japanese: on-reading クウ (kū) for compounds — 空気 (kūki, air — one of the foundational Japanese vocabulary words, also see 気 entry), 空港 (kūkō, airport), 空間 (kūkan, space), 航空 (kōkū, aviation). Kun-readings: そら (sora, sky) — the everyday word for the visible sky; あ.く (a.ku, to be empty / vacant — verb); から (kara, empty as a state) — 空っぽ (karappo, empty/hollow), 空腹 (kūfuku, hungry — "empty stomach").

Memory aid: cave + tool = a hollowed-out emptiness — extending upward to become "sky".