The Easiest Way to Remember Closed vs Open Interval Notation

An oval () fits inside a rectangle [ ] with the same width and height.

Cross-posted from here.

It has come to my attention that people come up with a lot of different mnemonics for remembering closed vs open interval notation.

The purpose of this post is to share my own mnemonic. The mnemonic is as simple as this post is brief.

The open interval $(a,b)$ fits inside the closed interval $[a,b].$ An oval $()$ fits inside a rectangle $[\,\,]$ with the same width and height.

It even looks that way visually with inequalities. Literally, the physical text $a < x < b$ fits inside the text $a \leq x \leq b.$ The latter is just the former with some extra lines underneath.