# Why are there 10 base pair steps, not 16?

In a biochemistry course I'm taking, the lecturer emphasised that there are 10 possible base pair steps; I've included a screenshot of a slide stating this. This confuses me, because I cannot work out why it's not 16; why are the 6 greyed out in the image not allowed (for example, CG stacked on CG counts, but GC stacked on GC doesn't)? TA|AT is listed as distinct from AT|TA, so it's not that the greyed ones are redundant. I've found a couple of papers mentioning that there are 10 base steps, but not why. (I can't get hold of the lecturer.)

Thanks in advance! • CG stacked on CG and GC stacked on GC are the same step with the chain turned by $180^\circ$, so there is no need to count it (and other similar steps) twice. Dec 22, 2020 at 11:09
Thus, if we consider a base pair step WX|YZ, we have Y folowing W on the positive sense strand (W->Y), and X following Z in the negative sense (antisense) strand (Z -> X). This means that, if we labeled the strands differently, we would have Z->X on the positive strand and W->Y on the negative one, i.e., we would have base pair step ZY|XW. I.e., the general symmetry rule is $$WX|YZ \rightarrow ZY|XW$$