Triangles have 3 sides.
Squares are 4-sided.
So, have you wondered why we say 3 square meals?
The phrase may have originated from the period when the Royal Navy’s meals were served on square wooden plates.
It may also have originated from the word SQUARE, which means proper (and honest). Thus, a square meal is a proper, satisfying meal. It also means, in this context, a balanced and healthy meal regimen, if we consider eating all three meals in a day.
