When it comes to labeling the 16384 columns of an Excel 2007 worksheet, our alphabet with its measly 26 letters is simply not up to the task. To make up the difference, Excel first doubles the letters in the cell’s column reference so that column AA follows column Z, after which you find column AB, AC, and so on. After that, triples the characters so that column AAA follows column ZZ, after which you get column AAB, AAC, and the like.

At the end of this letter tripling, the 16384th and last column of the worksheet ends up being XFD so that the last cell in the 1048576th row has the cell address XFD1048576.