A slider for double-cylinder circular knitting machines which comprises an elongated laminar body (12) which has a first longitudinal side (13) which is meant to be rested on the bottom (80a) of an axial slot (80) formed in the curved surface of the lower needle cylinder (81) or in the curved surface of the upper needle cylinder (81) and at least two heels (15,16) which protrude transversely to the longitudinal extension of the laminar body (12) on the second longitudinal side (14) of the laminar body (12) that lies opposite the first side. The laminar body (12) has, proximate to one of its longitudinal ends, a hook (17) for engagement with a needle (8). A first one (15) of the two heels (15,16) is arranged on a portion of the laminar body (12) which can move with respect to the remaining part of the laminar body (12) in a direction which has a component which is transverse with respect to the longitudinal extension of the laminar body (12) for the transfer of the first heel (15) from an inactive position to an active position, or viceversa. The first heel (15) is spaced from the first side (13) of the laminar body (12) more in the active position than in the inactive position.