Answer: straightedge

AnswerCrossword Clue
STRAIGHTEDGELine-drawing tool
STRAIGHTEDGETool for drawing a perfect line
straightedgeA bar with one accurately straight edge, used for testing whether something else is straight
straightedgea bar or piece of material (as of wood, metal, or plastic) with a straight edge for testing straight lines and surfaces or for cutting along or drawing straight lines