Why?

Take four slats, one of length a, another of length b and two of length c and join them as indicated in the figure below.

The points represent swivels, which means that the slats can turn freely around them. If the black swivels are fixed to some basis, like a cardboard, in such a way that the distance between them is equal to b, then the swivel P will move along a straight line or more precisely, along a segment of a straight line.

If you are not convinced, fix a pencil in P. You will see it for yourself.

Very well, but why is that so?

The answer.