Excel column numbering

I was working with a wide spreadsheet from a client the other day and I had to convert between Excel column labels and column numbers. I had never paid attention to how Excel labels columns and implicitly thought it was base 26 using letters rather than digits. But then I realized that’s not right. Excel labels columns A through Z, then AA trough AZ, then BA through BZ, etc. If this is base 26, then does A correspond to 0? That could work for A through Z, but then what about AA? Then you’d have to say the first A corresponds to 26 but the second A corresponds to 0. Does Z correspond to 0? If so then the column numbers would be 1 through 25, followed by 0, then 27. And it would mean that columns ZA through ZZ are the same as A through Z. In fact nothing in Excel column labeling corresponds to 0. The labels cannot be interpreted as a positional number system. There’s a name for this kind of number system: bijective base 26. The idea is ancient, but the name was coined recently. It has…