Trillion-Cut

Three sides, three corners, and one flat edge that sits tight against another stone — which is why trillions are the classic flankers beside a center stone, and why they are bought in pairs. Sides run straight for a crisp triangle or bow gently outward for a fuller one; in paler material, check the middle is not windowing.

Standard millimeter sizes, ready to set, so a pair takes the same seat either side of a three-stone mounting.