# Blackboard bold

"Doublestrike" redirects here. For other uses, see Double Strike (disambiguation).
An example of blackboard bold letters.

Blackboard bold is a typeface style that is often used for certain symbols in mathematical texts, in which certain lines of the symbol (usually vertical or near-vertical lines) are doubled. The symbols usually denote number sets. One way of producing Blackboard bold is to double-strike a character with a small offset on a typewriter.[1] Thus they are referred to as double struck.

## Origin

In some texts these symbols are simply shown in bold type: blackboard bold in fact originated from the attempt to write bold letters on blackboards in a way that clearly differentiated them from non-bold letters i.e. by using the edge rather than point of the chalk. It then made its way back in print form as a separate style from ordinary bold,[1] possibly starting with the original 1965 edition of Gunning and Rossi's textbook on complex analysis.[2]

## Rejection

Some mathematicians, therefore, do not recognize blackboard bold as a separate style from bold: Jean-Pierre Serre, for example, uses double-struck letters when writing bold on the blackboard,[3] whereas his published works consistently use ordinary bold for the same symbols.[4] Donald Knuth also prefers boldface to blackboard bold, and consequently did not include blackboard bold in the Computer Modern fonts he created for the TeX mathematical typesetting system.[5]

The Chicago Manual of Style in 1993 (14th edition) advises: "blackboard bold should be confined to the classroom" (13.14) whereas in 2003 (15th edition) it states that "open-faced (blackboard) symbols are reserved for familiar systems of numbers" (14.12).

## Encoding

TeX, the standard typesetting system for mathematical texts, does not contain direct support for blackboard bold symbols, but the add-on AMS Fonts package (`amsfonts`) by the American Mathematical Society provides this facility; a blackboard bold R is written as `\mathbb{R}`. The `amssymb` package loads `amsfonts`.

In Unicode, a few of the more common blackboard bold characters (C, H, N, P, Q, R and Z) are encoded in the Basic Multilingual Plane (BMP) in the Letterlike Symbols (2100โ214F) area, named DOUBLE-STRUCK CAPITAL C etc. The rest, however, are encoded outside the BMP, from `U+1D538` to `U+1D550` (uppercase, excluding those encoded in the BMP), `U+1D552` to `U+1D56B` (lowercase) and `U+1D7D8` to `U+1D7E1` (digits). Being outside the BMP, these are relatively new and not widely supported.

## Usage

The following table shows all available Unicode blackboard bold characters.

The symbols are nearly universal in their interpretation, unlike their normally-typeset counterparts, which are used for many different purposes.

The first column shows the letter as typically rendered by the ubiquitous LaTeX markup system. The second column shows the Unicode codepoint. The third column shows the symbol itself (which will only display correctly on browsers that support Unicode and have access to a suitable font). The fourth column describes known typical (but not universal) usage in mathematical texts.

LaTeX Unicode (Hex) Symbol Mathematics usage
$\mathbb{A}$ `U+1D538` ๐ธ Represents affine space or the ring of adeles. Sometimes represents the algebraic numbers, the algebraic closure of Q (โ or โ, although Q is often used instead). It may also represent the algebraic integers, an important subring of the algebraic numbers.
`U+1D552` ๐
$\mathbb{B}$ `U+1D539` ๐น Sometimes represents a ball, a boolean domain, or the Brauer group of a field.
`U+1D553` ๐
$\mathbb{C}$ `U+2102` โ Represents the set of complex numbers.
`U+1D554` ๐
$\mathbb{D}$ `U+1D53B` ๐ป Represents the unit (open) disk in the complex plane (for example as a model of the Hyperbolic plane), or the decimal fractions (see number).
`U+1D555` ๐
$D\!\!\!\!D$ `U+2145` โ
$d\!\!\!\!d$ `U+2146` โ May represent the differential symbol.
$\mathbb{E}$ `U+1D53C` ๐ผ Represents the expected value of a random variable, or Euclidean space, or a field in a tower of fields.
`U+1D556` ๐
$e\!\!e$ `U+2147` โ Sometimes used for the mathematical constant e.
$\mathbb{F}$ `U+1D53D` ๐ฝ Represents a field. Often used for finite fields, with a subscript to indicate the order. Also represents a Hirzebruch surface or a free group, with a subset to indicate the number of generators (or generating set, if infinite).
`U+1D557` ๐
$\mathbb{G}$ `U+1D53E` ๐พ Represents a Grassmannian or a group, especially an algebraic group.
`U+1D558` ๐
$\mathbb{H}$ `U+210D` โ Represents the quaternions (the H stands for Hamilton), or the upper half-plane, or hyperbolic space, or hyperhomology of a complex.
`U+1D559` ๐
$\mathbb{I}$ `U+1D540` ๐ Occasionally used to denote the identity mapping on an algebraic structure, or the set of imaginary numbers (i.e., the set of all real multiples of the imaginary unit), or the ideal of polynomials vanishing on a subset.
`U+1D55A` ๐
$i\!i$ `U+2148` โ Occasionally used for the imaginary unit.
$\mathbb{J}$ `U+1D541` ๐ Sometimes represents the set of irrational numbers, R\Q (โ\โ).
`U+1D55B` ๐
$j\!\!j$ `U+2149` โ
$\mathbb{K}$ `U+1D542` ๐ Represents a field, typically a scalar field. This is derived from the German word Kรถrper, which is German for field (literally, "body"; cf. the French term corps). May also be used to denote a compact space.
`U+1D55C` ๐
$\mathbb{L}$ `U+1D543` ๐ Represents the Lefschetz motive. See motives.
`U+1D55D` ๐
$\mathbb{M}$ `U+1D544` ๐ Represents the monster group. The set of all m-by-n matrices is denoted ๐(m, n).
`U+1D55E` ๐
$\mathbb{N}$ `U+2115` โ Represents the set of natural numbers. May or may not include zero.
`U+1D55F` ๐
$\mathbb{O}$ `U+1D546` ๐ Represents the octonions.
`U+1D560` ๐
$\mathbb{P}$ `U+2119` โ Represents projective space, the probability of an event, the prime numbers, a power set, the positive reals, the irrational numbers, or a forcing partially ordered set (poset).
`U+1D561` ๐ก
$\mathbb{Q}$ `U+211A` โ Represents the set of rational numbers. (The Q stands for quotient.)
`U+1D562` ๐ข
$\mathbb{R}$ `U+211D` โ Represents the set of real numbers.
`U+1D563` ๐ฃ
$\mathbb{S}$ `U+1D54A` ๐ Represents the sedenions, or a sphere.
`U+1D564` ๐ค
$\mathbb{T}$ `U+1D54B` ๐ Represents a torus, or the circle group, or a Hecke algebra (Hecke denoted his operators as Tn or ๐โ), or the Tropical semi-ring, or twistor space.
`U+1D565` ๐ฅ
$\mathbb{U}$ `U+1D54C` ๐
`U+1D566` ๐ฆ
$\mathbb{V}$ `U+1D54D` ๐ Represents a vector space or an affine variety generated by a set of polynomials.
`U+1D567` ๐ง
$\mathbb{W}$ `U+1D54E` ๐ Represents the set of whole numbers (here in the sense of non-negative integers), which also are represented by โ0.
`U+1D568` ๐จ
$\mathbb{X}$ `U+1D54F` ๐ Occasionally used to denote an arbitrary metric space.
`U+1D569` ๐ฉ
$\mathbb{Y}$ `U+1D550` ๐
`U+1D56A` ๐ช
$\mathbb{Z}$ `U+2124` โค Represents the set of integers. (The Z is for Zahlen, which is German for "numbers".)
`U+1D56B` ๐ซ
`U+213E` โพ
`U+213D` โฝ
`U+213F` โฟ
`U+213C` โผ
`U+2140` โ
`U+1D7D8` ๐
`U+1D7D9` ๐ Often represents, in set theory, the top element of a forcing partially ordered set (poset), or occasionally the identity matrix in a matrix ring. Also used for the indicator function.
`U+1D7DA` ๐
`U+1D7DB` ๐
`U+1D7DC` ๐
`U+1D7DD` ๐
`U+1D7DE` ๐
`U+1D7DF` ๐
`U+1D7E0` ๐
`U+1D7E1` ๐ก

In addition, a blackboard-bold Greek letter mu (not found in Unicode) is sometimes used by number theorists and algebraic geometers (with a subscript n) to designate the group (or more specifically group scheme) of n-th roots of unity.[6]