In most cases, you pluralize a single letter used as a noun by simply adding “s” to the end. There are some exceptions based on the capitalization of the letter and the style guide being used.
Here’s what you need to know about the proper way to write the plural of a single letter and the exceptions to those rules.
Forming the Plural of a Letter or Number
Under regular circumstances and according to standard style guides like the Chicago Manual of Style, there is no need to use an apostrophe when writing an uppercase letter or a number as a plural noun. The plural can be formed simply by adding “s” to the letter or number.
Matthew got several Cs on his report card this semester.
Why are there so many 7s in this math problem?
In these cases, it is relatively clear that “Cs” are multiple instances of the letter “C.” The same is true for the instances of the number “7.” Adding an apostrophe would actually make the sentence harder to read, because it may indicate possession by the letter or number where there is none.
Forming the Plural of a Lowercase Letter
Because the use of lowercase letters as nouns is relatively rare – often as a part of a colloquialism or common phrase – they tend to follow a different rule. To form the plural, you would add an apostrophe and an “S” to the end of the letter.
Mind your p’s and q’s.
Dot your i’s and cross your t’s.
This helps the lowercase letter stand out as a noun rather than appearing as a typo or a different word entirely.
Exceptions and Style Preferences
The above rules come from generally accepted best practices from multiple style guides. That being said, some style guides have exceptions to these rules, particularly when it comes to clarification.
For instance, in many guides, the letters “A,” “I,” “M,” and “U” are pluralized by adding an apostrophe and an “s,” whether they are capital letters or lowercase letters.
The teacher was pleased with all of the A’s from her assignment.
This is because they would otherwise form a different word or, in the case of “M,” abbreviation, which could lead to confusion about the sentence’s meaning.
There are also cases where the style guide allows for the interchangeable use of apostrophes when pluralizing letters and numbers.
The 1950s were a strange time.
The 1950’s were a strange time.
There is no such thing as UFOs.
There is no such thing as UFO’s.
If this is the case, you just need to be consistent with which pluralization option you choose to use. Choose a style the first time you use a letter or number as a noun and refer to that use whenever the issue rises again within a piece. Don’t switch between styles from paragraph to paragraph; this will cause your writing to appear inconsistent and piecemeal.
As with any rules of punctuation and capitalization, if you’re unsure what format to use, consult your style guide to see whether these exceptions apply to your writing. Every guide is different in some ways, which means that you should have it with you to reference when you’re writing a piece intended to follow it.
Single letters used as nouns are relatively rare, making their pluralization rules harder to learn and remember. Luckily, their rarity also means that they offer more forgiving formatting. As long as you choose a single, consistent style and stick with it, it’s very difficult to go wrong.