Swing trees were used with draught horses. These levers yoked the horse to the plough so that the horse would draw from a single point and the load would be evenly distributed.
Swing trees were commonly made from wood, but iron was also used. In this case, care had to be taken to ensure it was not too heavy for the horses.
Swing trees have also been known as swingle-trees, whipple-trees, draught-bars or bars. The same sort of mechanism is used in windscreen wipers, so that the force of the arm is distributed evenly across the windscreen.