Add is a verb: it means to put something together with something else, as in "add 2 and 2". Ad is a noun: it is short for advertisement, as in "a TV ad". If you could write "advertisement" in its place, write ad; otherwise, write add.
Add and ad belong on the long list of English words that are easy to confuse. They are spelled almost the same and pronounced exactly the same: they are homophones. That can make it hard to know which one to use.
To make matters worse, there is also ADD, the abbreviation for attention deficit disorder, a medical condition. The term is now considered outdated (the medical term today is ADHD), but it is still used. When it is, it is written entirely in capital letters, because it is an abbreviation.
This article explains the difference between add and ad and gives you a few tips to tell them apart. You may also be interested in a while vs. awhile.
What add means
Add is a verb. It means to put something together with something else, so that the size, number, or amount grows. It can also mean to include something extra, or to say one more thing ("She added that she would be late").
Examples of add in a sentence:
- The teacher asked Maria to add the two fractions on the board.
- I will add you to my list of contacts.
- No matter what you do, when you add 2 and 2, you always get 4.
So when you put numbers or quantities together, or include extra things, use add.
What ad means
The easiest way to remember ad is to think of a commercial. Ad is short for advertisement, and unlike add, it is a noun. The short form has been in use since the 1840s. It is fine to use ad in everyday writing, but in formal writing it is better to spell out the whole word, advertisement.
Examples of ad in a sentence:
- My friends and I were blown away by the new ad for the video game.
- The political ad was very negative and turned many voters off.
- Every time I see that ad on TV, I start to dance and laugh.
How to tell add and ad apart
- Ad means a commercial, because it is short for advertisement. If you can replace the word with "advertisement", write ad.
- Add is part of addition: whenever you think of addition, think of its first three letters, add.
- Add has an extra d, as if a letter had been added to it.
Because both are real words, a spell checker that only compares each word with a dictionary cannot flag a mix-up. The AI Spell Checker reads whole sentences, so it can also catch a real word in the wrong place, such as "ad" where you meant "add".
For another pair that is easy to mix up, read principle vs. principal.