Corner triangles are triangles that “live” in the corners of the quilt (or a quilt block). They are often used when squares are set on point. To help keep the quilt edges flat, these triangles should have the straight of grain and cross grain located on the legs (shorter sides) of the triangle. The bias should be located on the hypotenuse (longest side). It is cut differently than a side triangle (1/4 square triangles) because of this bias placement. Another name for a corner triangle is “half square triangle”. You will find more info, including a video, under Setting Triangles (link is below photo).