SAS is Side-Angle-Side. It means that we have two sides of one triangle congruent to the corresponding two sides of the other triangle, and the included angle of the first triangle congruent to the corresponding included angle of the second triangle.
In the first triangle, the base sides are marked congruent. The side that connects the triangles is congruent to itself. This is two sides; the right angle falls between them, making it the included angle. This is SAS.
