In a simple terms - They're not.
Avoid triangles if you can. You are better off with a really curved rectangle shade.
If you must have a triangle, (and they are an ok option in some situations) think about coverage. They may look good on paper or in your head, but in reality they do not do much.
Find a balance between aesthetics and the practical shade provided.
Basic maths- a triangle is half of a square. The investment required for a triangular shade sail is not half that of a square. Therefore you would be better off investing in a rectangle and cutting a bit of it off. The ratio of shade to shape is a number that I do not know but I know its way in favour of the rectangle or square.
If you must have a triangular shade sail…..
Design your triangular shade sail to have some angle on it, fall, slope whatever you call it. Two high points, 1 low or 1 high and two low. Never install a flat sail. It will only ever lead to dramas. Plus it looks ordinary. (You can not get twist with 3 connections points….you can with 4…just saying)
Make the 3 connection points, either equal distance apart, or, have 1 or 2 sides fixed.
Use a single or 2 direct connections to get more coverage. Just make sure you still have plenty of pre-tension on your sail.
Get it custom made by someone that knows what they are doing. If you see a drawing with straight lines between the connection or fixing points, then find someone else. You can have straight edges on a shade sail, it just costs a fortune in computer modelling, engineering and fabrication resources.
Have a design where multiple triangular shade sails are overlapping one another, or at least working together overall.
Do not install a triangular shade sail yourself. It’s guaranteed to look shit.
Make sure to call or email if you have any questions or want to enquire