Several parts combine into one character.

線 is a phonetic-semantic compound: 糸 (silk thread radical) + 泉 (quán, "spring / fountain" — providing the sound). The composite reads as "thread that flows continuously like water from a spring" = thread, line, and by extension every linear concept. From this concrete textile origin grew an extraordinary semantic spread covering geometry, transportation, telecommunications, and visual perception.

Korean reading "seon." Saturates technical vocabulary across multiple disciplines: 直線 (jikseon, straight line — geometry), 曲線 (gokseon, curve), 路線 (noseon, route / line — public transit), 無線 (museon, "no-line" = wireless), 有線 (yuseon, "has-line" = wired / cable), 電線 (jeonseon, electrical wire), 視線 (siseon, "sight-line" = gaze / line of sight — also used metaphorically for "public attention"), 境界線 (gyeonggye-seon, boundary line). Korean technical writing across math, physics, transit, and telecom is built on 線.

Mandarin xiàn, 4th tone (simplified 线). 线 (xiàn, line / thread), 直线 (zhíxiàn, straight line), 线路 (xiànlù, route), 电线 (diànxiàn, electric wire), 路线 (lùxiàn, route / itinerary), 线索 (xiànsuǒ, "thread-clue" = lead / clue in detective work — Chinese investigative vocabulary uses the thread metaphor more vividly than Korean or Japanese, treating clues as threads to be followed).

Japanese on-reading セン (sen) — 直線 (chokusen, straight line), 路線 (rosen, route), 新幹線 (Shinkansen, "new-trunk-line" = the famous Japanese bullet train system), 有線 (yūsen, wired), 無線 (musen, wireless), 視線 (shisen, gaze). Japanese train and subway lines all carry 線 in their names: 山手線 (Yamanote-sen, the loop line around central Tokyo), 中央線 (Chūō-sen, Central Line), 新幹線, 銀座線 (Ginza Line), 千代田線 (Chiyoda Line). For commuters in Japan, 線 is part of daily address and direction — "what line do you take?" 何線で行きますか?

Memory aid: thread (糸) flowing like water from a spring (泉) — continuous, extending, line.