The floor of y is the greatest integer less than or equal to y and is denoted ⌊y⌋.

Similarly, the ceiling of y is the greatest greater than or equal to y and is denoted ⌈y⌉.

Both of these notations were introduced by Kenneth Iverson.

Before Iverson’s notation caught on, you might see [x] for the floor of x, and I don’t know whether there was a notation for ceiling.

There was also a lack of standardization over whether [x] meant to round x down or round it to the nearest integer.

Iverson’s notation caught on because it’s both mnemonic and symmetrical.

Iverson also invented the notation of using a Boolean expression inside square brackets to indicate the function that is 1 when the argument is true and 0 when it is false.

I find this notation very convenient.

I’ve used it on projects for two different clients recently.

Here’s an equation from Concrete Mathematics using all three Iverson notations discussed here: ⌈x⌉ – ⌊x⌋ = [x is not an integer].

In words, the ceiling of x minus the floor of x is 1 when x is not an integer and 0 when x is an integer.

Related links Four handy notations Manipulating sums Notation as Tool of Thought by Kenneth Iverson.