Now that I’ve used a line of Fray-Check along the line I’m about to cut, you can see more clearly how the straight line isn’t actually all that straight. (PS, do use the Fray-Check, unless you’re thoroughly married to the idea of making the entire garment by entirely non-modern methods. This will stop it fraying no matter what you do to it, between the day you cut and the day you’re ready to sew.)