Two readers recently asked whether they need to repeat a number in parentheses after they write out the word.
Note that I did not write two (2) readers.
Putting the number in parentheses after the word is unnecessary and no style guide that I'm aware of calls for it. It has a sense of legalese to it, but from what I can tell, it's not even required in legal writing anymore. Garner's Modern American Usage says it was originally done in legal writing to prevent fraudulent alterations. I guess if you had to alter both the word and the numeral, it would be harder than if you had to alter just one or the other.
That’s why you sometimes see a numeral in parentheses after a number that is written out—it is a relic of legal writing, but it’s not something you need to include in your writing today.