If the butt of the cue is elevated above the horizontal axis when the cue ball is struck with english, it will curve. When the cue ball curves, the shot is usually referred to as either a curve shot or a mass shot.
Some people differentiate between curve and mass shots by how much the ball actually curves.
Generally, small curves are curve shots and the term mass is reserved for shots where the cue needs to be elevated over the shoulder to nearly 90 degrees to make the cue ball curve radically. For our purposes, we will use the term mass anytime the cue ball curves. During the path of a mass , the cue ball does not curve the entire length of the shot. Examining the mass shot, we can separate the entire path into 3 sections. For those who are not yet able to judge this kind of shot, there is a system to reduce the guess work needed in aiming a mass .