In its simplest form the carriage holds the tool bit and moves it longitudinally (turning) or perpendicularly (facing) under the control of the operator. The operator moves the carriage manually via the "handwheel" (5a) or automatically by engaging the feed shaft with the carriage feed mechanism (5c). This provides some relief for the operator as the movement of the carriage becomes power assisted. The handwheels (2a, 3b, 5a) on the carriage and its related slides are usually calibrated, both for ease of use and to assist in making reproducible cuts. The carriage typically comprises a top casting, known as the saddle (4), and a side casting, known as the apron (5).