Why do we usually use the word "square" when describing or referring to three meals in a day?

Some countries say three 'round' meals. Why don't they call them squares too, I dunno.

There may be other origins, but the one I am familiar with is this one. In good weather, crews' mess was a warm meal served on square wooden platters. It refers to crews on old sailing vessels. The Army promises nothing more than 3 squares and a cot.

It's my understanding that the round plates we use today were once square-shaped "tranches" upon which food was slopped.

People in the shipping industry (when that really meant sailing on a ship like you see in Pirates of the Caribbean) would advertise "3 square meals a day" as part of the benefit of signing on to sail for them. It meant you'd get 3 "proper" meals -- plates-ful, rather than the one meal a day if you were lucky.

the 4 corners of the food group

because it's supposed to have the 4 main food groups... 4 "corners" make a square

meat

milk

vegetables

bread

on the square

square deal

in older slang it meant solid, honest.