 Many experienced colonoscopists recognize that small polyps seen incidentally but not removed during insertion are sometimes quite difficult to find during withdrawal. The reason for this observation is probably because the colon is in a very different anatomical conformation during endoscope insertion and withdrawal. During insertion, the colon is in its natural conformation in which the sigmoid and transverse colon has several sharp bends or flexures, and the overall length has not yet been shortened. In this phase, the colon is often significantly stretched because of the formation of loops and bends in the colonoscope. This greatly affects the conformation of the colonic wall visualized proximal to the instrument tip. During withdrawal, the colon is shortened and pleated over the colonoscope, with successive regions of the colon being inspected as they slip off the end of the instrument. Thus, segments of visualized colon are often much straighter during withdrawal than during insertion. The insertion and withdrawal phases, therefore, expose somewhat different sections of the mucosal surface to the colonoscope and inspection on insertion and withdrawal are, quite possibly, complementary. 