A horological mechanism is the organ or the fundamental element of an instrument watch. It assures the main function that is to “tell time”. This mechanism results from a mechanical assembly of pieces which some of them can make a movement in relation to each other. Designated as a primary mechanism, it includes other mechanisms more elementary, at lower levels, just as the driving force, the cog, the escapement, etc. This mechanism can be observed at the case back of RESERVOIR’s watches models. This is the progress of different horological mechanisms that allow the evolution of the watchmaking science.