To form the plural of a single letter, add a lowercase s. Add a lowercase s with an apostrophe only if you form a word when you add the s without the apostrophe. For example, if you form the plural of the letter I or the letter A without an apostrophe, you create the words Is or As. Note, however, that even with the apostrophe, usage such as I’s can confuse the reader. When possible, rewrite the sentence to eliminate the need to use a plural single letter.