Lets establish our givens (please correct me if any of my givens are incorrect, for I am a physics noob.) 1.) A proton is made up of two up quarks and one down quark. 2.) An antiproton is made up of one up quark and two down quarks. 3.) A neutron is made up of one up quark and two down quarks. 4.) An antineutron is made up of two up quarks and one down quark. The four of these fermions have one of two makeups: Either a 1-up and 2-down quark setup, or a 1-down and 2-up setup (the latter being neutrons and antiprotons and the first being antiprotons and neutrons.) If all of this is so, then what is the difference between a proton and an antineutron, and what is the difference between a neutron and an antiproton? My guess would be that it has something to do with the rate of spin, but don't all fermions have spin rates of 1/2? THIS QUESTION IS DRIVING ME CRAZY AND NONE OF MY TEACHERS CAN ANSWER IT.