Construction: Copying a Segment

Say you're given a segment $AB$, and you want to make a segment congruent to $AB$ starting at point $C$. You do so by putting the vertex of your compass at $A$ and the pencil part at $B$, as if you're going to draw a circle with radius $AB$. Then, without changing the radius of your compass, move the vertex part to $C$ and draw a circle. Now use your straightedge to draw one of the radii from $C$, labeling the other endpoint $D$. Now you have a segment, $CD$, congruent to $AB$.