Cap for a container, comprising a side wall extending about an axis and a transversal wall arranged at an end of the side wall, a separation line being provided on the side wall for defining: - a retaining ring intended to remain anchored to a neck of the container, which is configured to internally engage with an enlargement of the neck; - a closure element which can removably engage the neck, so as to open or close the container; wherein the separation line extends about the axis and is circumferentially interrupted so as to leave a joining portion, between the retaining ring and the closure element, which extends circumferentially for a respective angle, the cap (1; 101) also having - an incision line (21; 21') which extends transversally to the axis between the separation line and a free edge of the retaining ring, so that two connecting bands (29, 30) are defined between the separation line and the incision line (21; 21'), the two connecting bands joining the retaining ring to the joining portion; wherein - the connecting bands (29, 30) are deformable, for allowing the joining portion to rotate when the closure element is moved from a closed position to an open position, and wherein - the joining portion is externally provided with a projection which projects from the joining portion, characterized in that the projection is positioned between a separation plane in which the separation line lies, and the incision line (21; 21') so that, when the closure element is in the open position and the connecting bands (29, 30) keep the closure element connected to the retaining ring, an edge of the joining portion which, in the closed position, faces the retaining ring, is facing, in the open position, towards a rim of the neck and the projection is resting on the neck in said open position. - a retaining ring intended to remain anchored to a neck of the container, which is configured to internally engage with an enlargement of the neck; - a closure element which can removably engage the neck, so as to open or close the container; wherein the separation line extends about the axis and is circumferentially interrupted so as to leave a joining portion, between the retaining ring and the closure element, which extends circumferentially for a respective angle, the cap (1; 101) also having - an incision line (21; 21') which extends transversally to the axis between the separation line and a free edge of the retaining ring, so that two connecting bands (29, 30) are defined between the separation line and the incision line (21; 21'), the two connecting bands joining the retaining ring to the joining portion; wherein - the connecting bands (29, 30) are deformable, for allowing the joining portion to rotate when the closure element is moved from a closed position to an open position, and wherein - the joining portion is externally provided with a projection which projects from the joining portion, characterized in that the projection is positioned between a separation plane in which the separation line lies, and the incision line (21; 21') so that, when the closure element is in the open position and the connecting bands (29, 30) keep the closure element connected to the retaining ring, an edge of the joining portion which, in the closed position, faces the retaining ring, is facing, in the open position, towards a rim of the neck and the projection is resting on the neck in said open position.