Several parts combine into one character.

細 is a phonetic-semantic compound: 糸 (silk thread) on the left plus 田 (today read as "field," but originally a variant of 囟 — the soft spot on a baby's skull / crown of the head). The earliest pictographic image was thus "fine threads on a baby's skull-fontanelle" — the most delicate, gossamer fineness imaginable. From that intimate concrete image grew the abstract sense "thin, fine, detailed, minute." The 糸 radical anchors the entire vocabulary of fineness: 細 (thin), 絹 (fine silk), 織 (weave), 紙 (paper, originally silk-thin sheets).

Korean reading "se." 仔細 (jase, detailed / careful — Korean adverb 자세히 means "in detail"), 細密 (semil, fine / detailed), 細胞 (sepo, cell — biological), 微細 (mise, microscopic / minute), 詳細 (sangse, detailed / specific), 細菌 (segyun, bacteria). The semantic chain "thin → precise → small → microscopic" carries the character through both everyday and scientific vocabulary.

Mandarin xì, 4th tone (simplified 细). 仔细 (zǐxì, careful / attentive), 细心 (xìxīn, "fine-hearted" = thorough / meticulous — a coveted personality trait in Chinese culture), 详细 (xiángxì, detailed), 细菌 (xìjūn, bacteria). Modern Mandarin keeps the meaning closely parallel to Korean and Japanese.

Japanese on-reading サイ (sai) — 詳細 (shōsai, details), 細胞 (saibō, cell), 微細 (bisai, microscopic). The two kun-readings split a subtle but important semantic distinction that trips up learners: ほそい (hosoi) means "thin / slender" applied to physical objects — 細い指 (hosoi yubi, slender fingers), 細い道 (hosoi michi, narrow path). こまかい (komakai) means "fine / detailed / particular" applied to abstractions — 細かい説明 (komakai setsumei, a detailed explanation), 細かいこと (komakai koto, small details / picky things). Mixing them up sounds odd: a "細い説明" is grammatically possible but unnatural — it suggests the explanation itself is physically thin. This split is one of the cleanest examples of Japanese assigning two different kun-readings to one kanji to mark physical vs abstract registers.

Memory aid: silk threads (糸) plus the soft crown of an infant's head (田 ← 囟) — fineness at its most tender.