This is the stepbystep, printable version. If you PRINT this page, any ads will not be printed.
See also the animated version.
After doing this  Your work should look like this 

Start with two given segments a and b. 

1. Mark a point P towards the left of the work area.  
2. Draw a long line from P to the right.  
3. Set the compasses width to the length of the segment a.
(Note: We can use the given segments in any order.) 

4. Without changing the compass width, place the point on P and make an arc across the line, creating point Q  
5. Set the compasses width to the length of the segment b.  
6. Without changing the compass width, place the point on Q and make an arc across the line to its left.  
Done. The segment PR has a length which is the difference between the lengths of a and b 