Three yards (108″, or about 2.75 m for the non-backwards metric-using nations of the world) is a standard length for belly dance veils . Thus, a 3 yard long half circle veil must have a radius of 54″. It must be cut from 54″ wide silk in order to really be a half circle. Some veils marketed as half circles would more accurately be described as half ellipse, because they’ve been cut from 45″ wide silk.
I prefer concise language when it comes to these curved-edge veils, but I’ve also caved to the standard of titling my half ellipses as half circles so they come up in searches (always with the appropriate caveat in the description though).
A Picture is Worth a Thousand Words:
It’s geometry…
A true half circle veil in the standard 3 yard length is a LOT of fabric. There is ~17% less fabric in the half ellipse, and these two shapes do not move exactly the same.
How important these is to your dancing depends entirely on your veil work style. If you like the way a half-circle or half-ellipse veil drapes when it’s tucked into your costume, but otherwise treat the veil much as you would a rectangular veil, a half-ellipse may be just fine.
However, the half circle has the same amount and weight of fabric radiating out from the center point, so every point around the edge is going to pull outward with the same centrifugal force when you do something like a fast barrel turn. This means the true half circle veil will tend to stay open in all its glory during these moves–even more so if you select heavier 8 momme silk for this purpose.
On the other hand, if you are of shorter-than-average height, a 54″ wide veil will require more strength and finesse to avoid stepping on it. For us shorties, a 54″ veil (whether semicircular or rectangular) will be dragging the ground when our arms are extended straight out from our sides.