The RIGHT function returns a Text value, not a numeric value, so ISNUMBER will not see the return from the RIGHT function as a number even if it is composed of all digits. However, if you involve a Text number in a mathematical operation, then Excel will convert the Text number to a real number in order to be able to complete the mathematical operation. So, if you add 0 or multiply by 1, you will convert the Text number to a real number so that ISNUMBER can see it as a number. So, making the change I show in red will get that part of your formula working correctly. I want to point out, however, that what I highlighted in blue is not guaranteed to return TRUE only for letters (what your thread title said you wanted)... the ISTEXT function will return TRUE for things like A12, 3B4, 56C, and the like. Also, since its argument is a Text function, ISTEXT will return TRUE even if the LEFT function returns all digits (see the first part of this response). You will need a different test to do what you want... let me think about it for a bit.

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