Definition
Straightedge
A straightedge is an unmarked straight tool used to draw lines, not to measure length.
Continue from this idea into connected AXIO math.
What it means
Place its edge beside two points, then draw along the edge. The tool controls straightness. Marks and numbers are not part of the construction, even when a ruler is used as the straightedge.
Remember it
Use the edge for straightness, not the numbers for distance.
Example
To draw the line through points P and Q, line up the straightedge so both points touch its edge, then draw along that edge past both points.
Watch for
A marked ruler can act as a straightedge only when its scale is ignored; using the marks to find a length is measurement, not straightedge construction.
Common mix-up
A straightedge draws a straight line. It does not by itself choose a distance, copy a length, or measure an angle.
Exact meaning
A straightedge is an unmarked tool with a straight edge, used in geometry to draw a straight line through given points.