### Theory:

Construct a line segment of length $$2$$ $$cm$$ using ruler and compasses.

Step 1: Draw a line $$l$$. Mark a point $$A$$ on a line $$l$$.

Step 2: Place the compasses pointer on the zero mark of the ruler. Open it to place the pencil point up to the $$2$$ $$cm$$ mark.

Step 3: Please see that the opening of the compasses has not changed, place the pointer on $$A$$ and swing an arc to cut the line $$l$$ and then mark the point as $$B$$.

$$AB$$ is a line segment of the required length.