straight edge 
noun
A bar with one accurately straight edge, used for testing whether something else is straight.I clamp a steel straight edge in a vise and just draw the surface over the steel edge a few times....
- They checked to see if the slab would easily wrap around the pipe and then used a straight edge to cut out an appropriate-sized rectangle from the clay.
- Measuring instruments, like levels and straight edges, are in that anomalous class of hand tools which do not directly affect the work piece, but only report on its status.
adjective chiefly US
(Especially among fans of hardcore punk music) having an ascetic or abstinent lifestyle: he’s so straight-edge that he won’t even take Tylenol when he has a headache