straight·edge /ˈstretˌɛʤ/
畫直線用尺
Straight·edge n. A board, or piece of wood or metal, having one edge perfectly straight, -- used to ascertain whether a line is straight or a surface even, and for drawing straight lines.
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straightedge
n : hand tool consisting of a flat rigid rectangular bar (metal
or wood) that can be used to draw straight lines (or test
their straightness)