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 \mathbb{C}, 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.
| symbol | set |
 | algebraics |
 | Booleans |
 |  -ball |
 | complex numbers |
 |  -dimensional complex projective space |
 |  -disk |
 |  -dimensional Euclidean space |
 | any field |
 | Gaussian integer |
 | quaternions, upper half-plane |
 |  |
 | hyperbolic plane |
 | integers |
 |  identity matrix |
 | natural numbers |
 | octonions |
 | primes |
 |  -dimensional real projective space |
 | rational numbers |
 | reals |
 | real numbers
in  dimensions |
 | real 
matrices |
 |  -dimensional real projective space |
 |  -sphere |
 |  -torus |
 | integers |
 | integers (mod
 ) |
 | negative integers |
 | positive integers |
 | nonnegative integers |
