The compound 'straight-line' is a transparent combination of two native English words, each with a long history. 'Straight' evolved from the Old English concept of being stretched taut, implying directness and absence of deviation. 'Line' originates from the Latin word for a linen thread, a quintessential symbol of a thin, continuous mark. When combined, the literal meaning of 'a direct, uncurving path' is immediately clear. This concrete geometric sense was later metaphorically extended into finance and accounting to describe a method of calculation that proceeds uniformly from one point to another, mirroring the visual simplicity of drawing a direct line between two values on a graph.

Structure: straight (direct) + line (path) - straight: From Middle English streght, streyt, from Old English streht (past participle of streccan 'to stretch'), of Germanic origin. In this compound, it functions as an adjective meaning 'direct, not curved or bent'. - line: From Middle English line, from Old English līne ('rope, series, row'), from Latin līnea ('linen thread, string, line'), from līnum ('flax'). In this compound, it functions as a noun meaning 'a continuous extent of length'.