Each pair of headstraps is composed of:
1. a universal bottom strap with an overhand knot which can be moved to micro-adjust the length (green 17.5 inches).
2. The top strap is measured from one top slit in your mask to the other, around the top/back of your head (see photo). The length of this very stretchy strap should be about 1-inch shorter than the length you measure.