Well, you say, that's not too difficult. To keep your interest heightened, some countries have produced stamps with perforations differing on three or even all four sides. New Zealand and some Australian states are notable in this regard. These stamps are described giving the top measurement first and then proceeding clockwise for the rest. Don't feel smug yet.
Some stamps have one perf.
We refer to stamp catalogues to help us know which stamp issues are so afflicted. The difficult perforation combinations are restricted to a relatively few issues. We are not required to worry about such things. We can enjoy the immense pleasures of stamp collecting without going in to such detail. Only those of us who are fascinated by such things need be involved with the intricacies of perforation measurements.