(i) Given AB whose length is not known.
(ii) Fix the compasses pointer on A and the pencil end on B. The opening of the instrument now gives the length of AB.
(iii) Draw any line l. Choose a point P on l. Without changing the compasses setting, place the pointer on Q.
(iv) Draw an arc that cuts l at a point R.
(v) Now place the pointer on R and without changing the compasses setting, draw another arc that cuts l at a point Q.
Hence, PQ is the required line segment whose length is twice that of AB.
