A. S. Besicovitch, On Arcs That Cannot Be Covered by an Open Equilateral Triangle of Side 1, The Mathematical Gazette, Vol. 49, No. 369 (Oct., 1965), pp. 286-288 ...

Trihexaflexagon is constructed by cutting a strip of paper so that it may be marked off in 10 equilateral triangles (A). The strip is folded backward along the line ab and turned over (B).