How can congruent segments and lengths be created with a tool with no marked measurements?

STEPS:
1. Using a straightedge, draw a reference line, if one is not provided.
2. Draw a dot on the reference line to mark your starting point for the construction.
3. Place the point of the compass on point A on the given figure.
4. Stretch the compass so that the pencil is exactly on B. Make a small arc through B.
(This small arc will show that you measured the length of the segment with your compass.)
5. Without changing the span of the compass, place the compass point on the starting point (dot) on the reference line and swing the pencil to create an arc crossing the reference line.
6. Label your copy.