Abstract: Master-keyed pin tumbler lock and relative master-key, where each pin has an inner window with an outer protuberance, which a spring acts upon and an inner projection, which the key acts upon, giving rise to the movement of the pin. The inner projections are at different heights and the pins are placed so that some move in one direction and others in the opposite direction, each pin having, in its inner window, a step placed on the opposite side to that of the inner projection. On one of the sides of the key there is a notch whose minimum width is the distance between the maximum level reached by the projections of the pins that move in one direction and the maximum level reached by those of the pins that move in the opposite direction; on the opposite side they have a double serration combined with the steps of the inner windows of the pins. For application in locks.