The website, Oxford Dictionaries, says the following about the use of the apostrophe: “The general rule is that you should not use an apostrophe to form the plurals of nouns, abbreviations, or dates made up of numbers: just add -s (or -es, if the noun in question forms its plural with -es).”
Which, if correct, means it is 'ABCs', as opposed to …. in an advert promoting a free Ladybird Books offer on the Daily Record website.
“Learn your ABC's….”, it says.
