|As far as I know, the only single letter that Excel will pick up as misspelled is "i". It will assume you meant "I" as in "I went to the store." As long as it is bringing up the dictionary, it will also offer you some other choices.|
If the spell checker assumed that every "single letter" was misspelled, it would flag sentences like this with multiple misspellings:
I got a D in Math class but a B in Science, so I guess my average is C.
There are also times when a letter might be used as a unit of measure, such as m for meter and x for an unknown value. In addition, a lettered bullet list would be flagged as a whole bunch of misspelled words.
In your case, "f" could be a misspelling of "of", "if" or "fa" so you wouldn't want to hardcode anything into the AutoCorrect list.
However, there is a workaround if you really need to flag single letters as misspellings.
1 - In Excel (assuming 2007 or later) Click File...Options...Proofing...AutoCorrect Options
2 - In the Replace box enter f
3 - In the With box enter ff
4 - Repeat this process for any other single letter you want to flag as a misspelling
Whenever you accidentally type one of the single letters that you set up in AutoCorrect it will be replaced with the e.g. ff, rr, dd, etc. Since ff, rr, dd, etc. are not in the Office dictionary, they will be flagged as misspelled words when you spell check the document.
Just keep in mind the the AutoCorrect options are universal to Office, not just Excel. If f will show up as ff in Excel, it's also going to show up as ff in Powerpoint, Word, etc.
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