What type of literary device is fair and square?

"Fair and square" isn't a specific literary device itself. It's a idiom, meaning a phrase with a figurative meaning that is different from the literal meaning of the words.

In this case, "fair and square" means honestly and justly, often in the context of a competition or game.

Here's why it's not a literary device:

* Literary devices are specific techniques used by authors to achieve a particular effect, like metaphors, similes, alliteration, etc.

* Idioms are simply phrases that have a commonly understood meaning different from the literal meaning of the words.

While idioms aren't literary devices, they can add color, vividness, and understanding to writing.