tres·tle 
(trĕsəl)
(trĕsəl)n.
1. A horizontal beam or bar held up by two pairs of divergent legs and used as a support.
2. A framework consisting of slanted braces and horizontal crosspieces supporting a bridge or causeway.
[Middle English trestel, from Old French, alteration of Vulgar Latin *trāstellum, trānstellum, diminutive of Latin trānstrum, beam; see TRANSOM.]