Co- comes from Latin and is short for "together, with, joint, common". Think co-operation: working together. If you really want to remember the difference, thin

Co- comes from Latin and is short for "together, with, joint, common". Think co-operation: working together. If you really want to remember the difference, think of co-itus. Itus comes from ire (to go), and the expression doesn't mean "going backwards".