Operators cab of tower cranes

In order to hook and unhook the loads, the operator usually works in conjunction with a signaller (known as a 'dogger', 'rigger' or 'swamper'). They are most often in radio contact, and always use hand signals. The rigger or dogger directs the schedule of  lifts for the crane, and is responsible for the safety of the rigging and loads. A tower crane is usually assembled by a telescopic jib (mobile) crane of greater reach (also see "self-erecting crane" below) and in the case of tower cranes that have risen while constructing very tall skyscrapers, a smaller crane (or derrick) will often be lifted to the roof of the completed tower to dismantle the tower crane afterwards.