The eighth proposition proves SSS congruence, or that if two triangles have corresponding sides equal, they must also have corresponding angles equal. This combines two methods that Euclid uses for proofs, superposition and contradiction. First, △DEF is superposed onto △ABC so that D coincides with A and DE lies up with AB. This means that E coincides with B. To prove that F lines up with C, it is assumed that F does not line up with C. However, this contradicts the previous proposition, therefore F must line up with C.