Do apostrophes used to show plurals of numbers and letters?

Answer 1

If you want. I've heard that an apostrophe with letters in numbers is an American thing. I've also heard that the Brits do it differently. I'm American and I don't use an apostrophe with letters and number as long as there is no confusion to the reader.

Examples:

1300s or 1300's

ABCs or ABC's

Both examples are correct. It is my opinion that there is little chance of the reader becoming confused.

Examples when lack of an apostrophe might confuse the reader:

Mind your p's and q's.

Always remember to dot your i's and cross your t's.

Answer 2:

Apostrophes are commonly, but erroneously, used in numbers such as calendar decades or centuries instead of using the correct form, e.g. 1800s or the 1970s. According to the Oxford Dictionary, numbers used in this format should not have an apostrophe. It is acceptable (though not a rule) to use it for individual numbers such as "count in 7's as far as 100". Though acceptable, it is unnecessary.

In some situations it is acceptable to use an apostrophe to clarify a plural for lower case letters only - again, the Oxford Dictionary states that it is incorrect to use it for capitals. ABCs is correct. ABC's is not.