A straightedge or straight edge is a tool used for drawing straight lines, or checking their straightness. If it has equally spaced markings along its length, it is usually called a ruler.

What is a straightedge in math?

An idealized mathematical object having a rigorously straight edge which can be used to draw a line segment.

What are examples of straight edge?

Most would agree that to be considered straight edge, you must abstain from consuming alcohol and drugs, smoking cigarettes, and engaging in sex outside of caring relationships, but many 'edge' individuals choose to abstain from other substances as well.