IF formula in Excel / Replace numbers with 'words'

Hey there,

I got the following table: see below

Now what I want to do, is to replace the numbers with words.

In words this would look something like this

FOR ROW Q:

If Value'Q' = 1 replace by A
If Value'Q' = 2 replace by B

FOR ROW R:

If Value'Q' = 1 and Value 'R' = 1 replace by aa
If Value'Q' = 1 and Value 'R' = 2 repalce by aa'
If Value'Q' = 2 and Value 'R' = 1 replace by bb

etc ...

I hope this explains it.

Cheers and thx a lot.

Philipp