A letter of the alphabet drawn with doubled vertical strokes is called doublestruck, or sometimes blackboard bold (because doublestruck characters provide a means of
indicating bold font weight when writing on a blackboard). For example, , ,
, , ,
.... Important sets in mathematics are commonly denoted using
doublestruck characters, e.g., C for the set of complex numbers,
Q for the rational numbers, R for the
real numbers,
for Euclidean n-space, and Z
for the integers.

The use of doublestruck characters is a relatively recent typesetting convention, with older book and manuscripts using either an unadorned capital letter (e.g., and in Peressini et al. 1998, pp. 1 and 5) or a
bold capital letter (e.g., , ,
and
in O'Neill 1966, pp. 3-5).

Doublestruck characters can be encoded using the AMSFonts extended fonts for LaTeX using the syntax \mathbbC, and typed in the Wolfram
Language using the syntax \[DoubleStruckCapitalC], where C denotes
any letter.

Many classes of sets are denoted using doublestruck characters. The table below gives symbols for some common sets in mathematics.