I'm reading something which highlights the Feynman rules for QED, and it talks about what happens if you go along a fermionic line. For example this picture: http://wikipremed.com/image_science_archive_68/010601_68/170750_298px-ComptonScattering-u.svg_68.jpg According to these notes, there would be a term from the incoming electron on the bottom left (call it A) then a term from the vertex (call it B) then the propagator (C) then the next vertex (D) and finally the outgoing electron (E). So the notes would then say to do that part you would have: EDCBA I get all that.. but what I don't understand is where to put the terms from the external photon lines. I would guess that it would be somewhere before or after the vertices. Can someone just tell me where they would go? Thanks.