X1 and X2 are on offense O1 and O2 on defense. Play until score or O1 and O2 gain possession and pass to O3 and O4. O3 and O4 are now on offense and X1 and X2 are now defense,(O1,O2 go back in line). Play until score or X1, X2 gain possession and give to X3 and X4. O3 and O4 are now on defense. Play continues perpetually in that pattern.