By folding an A4 sheet in thirds lengthwise, the intersections with the diagonal can be carried over to the[br]side to build a triangular network. We mark the diagonals of the small rectangles. We go up the lower left corner on the first crossing to the third, in the same way for the lower right corner.

The A4 format is 21cmx29.7cm or a ratio of [math] \sqrt {2} [/math]. So that the diagonal is of length [math] \sqrt {3} \times 21 cm [/math] and its third at [math] \frac {21} {\sqrt {3}} [/math], allows you to build rectangles of the format [math] \sqrt {3} [/math], and therefore equilateral triangles.