1) In a horse-drawn plough or carriage this is a cross-bar, pivoted at the middle, to which the traces are attached.
1618 the horse mill with the furniture a swingle tree & the linkes 5s 6d, South Cave
1639 three paire of traces & three Swineltres, Swinsty
1681 4 payre of hames with the traces and swengletrese, Lund. It derives from the verb to swing and this influenced some spellings: c.1742 stave body, raiths, swinging tree, Holmfirth. In some contexts the meaning is not absolutely clear, for it could also be a board on which flax and hemp were dressed.