Hi Guys. I'm hoping that you might be able to help me out with a question I have about anti-particles in Feynman diagrams. I'm drawing these again for the first time in over a decade because I'm teaching an elementary physics course that requires the students to draw diagrams for a few simple processes. The last time I drew these was when I was an undergraduate and so I don't really have any of the deep background knowledge to convince myself of the right answer. When I was at university I drew anti-fermions with arrows pointing in the opposite directions to the fermions. I have always assumed that this was one of the basic rules for constructing Feynman diagrams. I've checked this forum and there are plenty of posts ( like this one https://www.physicsforums.com/showthread.php?t=453948) that agree with this backwards anti-fermion construction. Likewise, the hyperphysics website, examples from the CERN education pages etc. all follow the same example. However, the folks that set the exam that my students have to sit have different ideas. All of the questions in their exam require both fermions and anti-fermions to have arrows pointing forward in time (page 12). The only mention I can find of this discrepancy in the exam information says simply 'some particle physicists write reverse the arrows for anti-particles." This seems quite unsatisfactory - as if the rules for constructing these things are a matter of personal taste - and in any event having them all in the same direction all of the time seems to make the whole excercise of drawing the arrows a bit pointless So my question is this... Are the directions of the fermion arrows required by underlying phyiscs (CPT symmetry? charge conservation?) to point in specific directions or is this just a convention that we are free to ignore at will?