#### 16.2.3 Blackboard bold ¶

Synopsis:

\usepackage{amssymb} % in preamble
...
\mathbb{`uppercase-letter`}

Provide blackboard bold symbols, sometimes also known as doublestruck
letters, used to denote number sets such as the natural numbers, the
integers, etc.

Here

\( \forall n \in \mathbb{N}, n^2 \geq 0 \)

the `\mathbb{N}`

gives blackboard bold symbol ℕ,
representing the natural numbers.

If the argument contains something other than an uppercase letter, you
do not get an error but you do get strange results, including
unexpected characters.

There are packages that give access to symbols other than just the
capital letters; look on CTAN.