A TT (Time Trial) bike and a Triathlon bike look very similar but are actually two very different bikes. Both have an almost vertical seat post and are very aerodynamic. The main difference is that for TT bikes they have to follow the UCI (Union Cycliste Internationale) rules where the rear triangle has to be a certain shape, the seat can not be past a certain position and the lengths and shape of the aero bars are also to a specification whereas the Triathlon bikes are not held to these rules. Please always check your bike fits in the rules of your competition.