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Webster 1913 Edition

Adj.

[L.

triquetrus

Three sided, the sides being plane or concave; having three salient angles or edges; trigonal.

Adj.

[L. triquetrus, from triquetra, a triangle.]

Three-sided; having three plane sides.

triquetrous ‎(comparative more triquetrous, superlative most triquetrous)

Having three corners or sides; triangular.
- 1658, Sir Thomas Browne, The Garden of Cyrus, Folio Society 2007, p. 172:
  the lithostrata or figured pavements of the ancients [...] consisted not all of square stones, but were divided into triquetrous segments, honey-combs, and sexangular figures, according to Vitruvius