I was taught the Windsor knot in the military, it is easy to tie, thus it's military use I'd assume. The Double Windsor has a nice finish and gives a the neat trianglular knot (so loved in the old clip ons), and a nice dimple in the top of the tie. As it's name states, it is a modified version of the Windsor knot and just adds an addition loop to firm it up. Although called a double which it is, it is less bulky and more balanced than the old Windsor knot which tends to look lopsided and bulky.