The perimeter of each sail is designed with a curve (also referred to as a gore) inwards towards the center of the sail to control the fabric tension.
Shade Sails also known as Sun Sails have reinforced webbing around the perimeter and marine stainless steel d-rings or “Delta” rings at each corner.
Years ago, people would hang up a tarp with straight sides and grommets in the corners over what they wanted to protect or shade an area.
The grommets in the corners of the tarp were attached to the mounting points and pulled tight.
The outside edges of the tarp would be tensioned tight, however the middle of the tarp would sag and flap in the wind.
So curves were put in the edges of the perimeter and once the corners were pulled, that would attempt to straighten out the curves in the sides, but they could not because they were attached to the fabric in the center of the shade sail.
This effect would in turn tighten up the fabric in the center of the shade sail. This eliminates the sagging or loose fabric like in the middle of the tarp with the grommets.
When properly tensioned, the fabric will not wrinkle, sag, or flap in the wind.