+1 on what Mtbwaterhouse said. It's the same as the holes a running back would take. 2,4,6, on the right side, 1,3,5,7 on the left side. The 8 and 9 holes are sweeps to the outside.
Holes 1 + 2 = A Gaps
Holes 3 + 4 = B Gaps
Holes 5 + 6 = C Gaps
Sweeps 8 + 9 = D Gaps
Hope this helped.