Constructing a Copy of a Given Line Segment

Constructing a copy of a given line segment:

To make a copy of `bar"AB"`.

Step 1: Given `bar"AB"` whose length is not known.

Step 2: Fix the compasses pointer on A and the pencil end on B. The opening of the instrument now gives the length of `bar"AB"`.

Step 3: Draw any line l. Choose a point Con l. Without changing the compasses setting, place the pointer on C.

Step 4: Swing an arc that cuts l at a point, say, D. Now `bar"CD"` is a copy of `bar"AB"`.