[xrisk]

Presumably the hypotenuse of a right angled triangle.

[gilleain]

Indeed. Related is the Ammann-Beenker tiling and it's connections to the 'Silver Ratio' of sqrt(2) + 1.

* https://en.wikipedia.org/wiki/Silver_ratio

* https://en.wikipedia.org/wiki/Ammann%E2%80%93Beenker_tiling

[hinkley]

Hypotenuse of a 1x2 unit right triangle, to be precise. By Pythagoras, the square root of any sum of squares can be drawn trivially with a compass and a straight edge. So 2, 5, 7, 10, 13, 17, etc