Triangles which are on equal bases and between the same parallels are equal to one another. Let $ABC$ and $DEF$ be triangles on the equal bases $BC$ and $EF$, and between the same parallels $BF$ and $AD$. I say that triangle $ABC$ is equal to triangle $DEF$.  <prf> Thus, triangle $ABC$ is equal to triangle $DEF$.Thus, triangles which are on equal bases and between the same parallels are equal to one another.