Nothing, a straightedge is a ruler. Straightedge is just another name for a ruler.
You can also draw straight lines with things such as the side of a folder or anything else with a flat edge.
A ruler can be classed as a straight edge.
Called a straight edge
A straight edge is basically another name for a ruler, or it could be anything that makes straight lines.
A straight edge
A geometrical construction can be made with with a straight edge and a compass.
A ruler is a straight edge
A ruler is a straight edge
A Ruler
A ruler can be classed as a straight edge.
A ruler or straight edge.
A ruler has markings, a straight edge does not. In "straight edge and compass" constructions, you are not allowed to mark the straight edge. Things that are impossible using only a straight edge and compass are possible if markings are allowed (for example, it's possible to trisect an angle using a marked straight edge).
Called a straight edge
Straight Edge
A straight edge
Like: "The edge of a ruler is straight" or "I walked straight to the store"
A straight edge is basically another name for a ruler, or it could be anything that makes straight lines.
It is a ruler with 12 inches on it