A butt cap will remain the exact measurement from point to point. a Protruding cap will not keep the exact measurement from point to point. As the name suggests, it protrudes over the start and end points. This means that two lines can start out being the same measurement, however, if one is made to have a Butt cap and the other a Protruding cap, the one that is a protruding cap will automatically become a little longer than the one that is a butt cap.