The invention describes an apparatus (10) for applying marking to perpendicular surfaces (15, 17) of an object (16) being conveyed along a line (11). A single marking head (12) is mounted on a positioning arm (14) which directs the marking head across the direction of the conveyor while applying marking to a first surface (15) of the object. The marking head is then rotated through 90° and held at one side of the conveyor so that a second, perpendicular, surface (17) of the object can be marked as the object is conveyed past the marking head. The invention uses the kinetic energy of the moving object to effect movement of the marking head.