Which instrument is used to draw straight lines perpendicular to a reference surface?

• A. Compass
• B. Level
• C. T-square
• D. Ruler

Answer: (C)

To draw a straight line at a right angle to a reference surface, you need a tool that fixes a 90-degree relationship to that surface. The T-square is designed for this purpose: its head sits firmly against the reference edge, and the blade is mounted perpendicular to that edge. When you align the head with the edge and draw along the blade, the line produced is perpendicular to the surface. This makes it the most straightforward and reliable option for getting lines that are square to the reference edge, such as borders or guides on a drawing sheet. Other tools don’t enforce that fixed 90-degree relationship in the same direct way, so they don’t guarantee perpendicularity to the surface.