AMSFonts package. Example:

usepackage [] amsfonts

This font is useful for representing sets like R (real numbers) or Z (integers). For instance:

"The set of real numbers are represented by " <> mathbb "R" <> "."

Or in monadic form:

"The set of real numbers are represented by " >> mathbb "R" >> "."

Note the use of overloaded strings.

Fraktur font.

Number sets