Key for a lock and a combination of a cylinder lock and a reversible flat key

A key for a lock, comprising a grip, a shaft having two opposite main faces and two opposite secondary lateral faces, at least a lever having a first arm for starting movement and a second arm for activating a tumbler element of the lock, said lever being pivoted to the shaft of the key with an oscillating axis thereof being transversal to the longitudinal axis of the shaft of the key, wherein said first lever arm and said second lever arm are located on a same main face of the shaft of the key, said lever being housed in a groove of said main face extending longitudinally along said shaft up to the end of the shaft opposite to the grip of the key, wherein in an idle position thereof, said lever is positioned entirely internally of said groove, and wherein the key further comprises releasable retaining means of said lever in an inactive position.