It's two triangles in one! This quilt ruler from Creative Grids allows you to cut half-square triangles and quarter triangles with ease.  
 
Use the 45-degree side to cut half-square triangles from strips up to 5-1/2 inches wide to create 5-inch finished half-square triangles. The 90-degree side can be used to cut quarter triangles that finish at a 5-inch quarter-square triange.
 
This ruler is easy to read with black and white markings printed right on the tool. It slides easily over the fabric until pressure is applied and the Creative Grids exclusive grip holds the fabric in place while cutting, eliminating slipping and miss-cuts!
 
Plus, the tool comes with fully illustrated step-by-step instructions and access to a how-to video.