内線

ないせん · naisen · JLPT N2 · noun

extension

📞 内 (inside) + 線 (line) = an inside phone line within one office, not an outside call

内 means inside and 線 means line or wire. A naisen is the internal line linking phones within one building or company, so calling a coworker's desk is a naisen call.

内 — enclosure 冂 + enter 入
内 originally depicted something entering (入) into an enclosed space (冂), giving the core sense of 'inside' or 'within'. The pictographic image of going into a covered area extended naturally to meanings like inner, internal, and inland. It contrasts with 外 ('outside').

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