Suppose we want to construct a triangle with sides 5 cm, 6 cm and 7 cm. We use compass and ruler. Start with the base, and mark a line of 5 cm, PQ below.
Open the compass to a length of 6 cm and draw an arc as shown below.
Now open the compass to a length of 7 cm and from the other end of the base, draw an arc as shown below.
The arcs so drawn cross at the third vertex, R, of the triangle.