The fact that that when both diagonals are drawn in on a square, four congruent 45-45-90 triangles are formed is discussed in this lesson.

Following this lesson, you'll know that when both diagonals are drawn in on a rectangle, two pairs of congruent isosceles triangles are formed.

Following this lesson, you'll be able to name a triangle based on the three vertices and the triangle symbol. It is 2 of 6 in the series titled "Triangle Notation."