A mechanism for writing instruments that includes a first element, which includes a guide extending between substantially parallel first and second edges and a second element, which includes a pin extending into the guide and is movable along the guide, where the pin has a given width along the transverse direction of the guide; and at least one locking device for the pin. The locking device includes a recess extending in the first edge between a first and a second end and an elastic member which extends opposite the recess from at least one first base integral with the first end of the recess.