 To those who decided to read the Elements carefully and see how the crucial terms are used, it became apparent that the account is both remarkably scrupulous in some ways and flawed in others. Straight lines arise almost always as finite segments that can be indefinitely extended, but, as many commentators noted, although Euclid stated that there is a segment joining any two points he did not explicitly say that this segment is unique. This is a flaw in the proof of the first congruence theorem (I.4) which says that if two triangles have two pairs of sides equal and the included angle is equal then the remaining sides of the triangles are equal.