Blackboard bold

Blackboard bold is a style of writing bold symbols on a blackboard by doubling certain strokes, commonly used in mathematical lectures, and the derived style of typeface used in printed mathematical texts. The style is most commonly used to represent the number sets , , , , and .
To imitate a bold typeface on a typewriter, a character can be typed over itself ; symbols thus produced are called double-struck, and this name is sometimes adopted for blackboard bold symbols, for instance in Unicode grapheme names.
In typography, a typeface with characters that are not solid is called inline, handtooled, or open face.

History

Traditionally, various symbols were indicated by boldface in print but on blackboards and in manuscripts "by wavy underscoring, or enclosure in a circle, or even by wavy overscoring".
Most typewriters have no dedicated bold characters at all. To produce a bold effect on a typewriter, a character can be double-struck with or without a small offset. By the mid-1960s, typewriter accessories such as the "Doublebold" could automatically double-strike every character while engaged. While this method makes a character bolder, and can effectively emphasize words or passages, in isolation a double-struck character is not always clearly different from its single-struck counterpart.
Blackboard bold originated from the attempt to write bold symbols on typewriters and blackboards that were legible but distinct, perhaps starting in the late 1950s in France, and then taking hold at the Princeton University mathematics department in the early 1960s. Mathematical authors began typing faux-bold letters by double-striking them with a significant offset or over-striking them with the letter I, creating new symbols such as
IR,
IN,
CC,
or ZZ;
at the blackboard, lecturers began writing bold symbols with certain doubled strokes. The notation caught on: blackboard bold spread from classroom to classroom and is now used around the world.
Loomis and Sternberg (1968).jpg|thumb|upright=1.25|A page from Loomis & Sternberg (1968), showing an early example of "blackboard bold" style R and C in a printed book]
The style made its way into print starting in the mid-1960s. Early examples include Robert Gunning and Hugo Rossi's Analytic Functions of Several Complex Variables and Lynn Loomis and Shlomo Sternberg's Advanced Calculus. Initial adoption was sporadic, however, and most publishers continued using boldface. In 1979, Wiley recommended its authors avoid "double-backed shadow or outline letters, sometimes called blackboard bold", because they could not always be printed; in 1982, Wiley refused to include blackboard bold characters in mathematical books because the type was difficult and expensive to obtain.
Donald Knuth preferred boldface to blackboard bold and so did not include blackboard bold in the Computer Modern typeface that he created for the TeX mathematical typesetting system he first released in 1978. When Knuth's 1984 The TeXbook needed an example of blackboard bold for the index, he produced using the letters I and R with a negative space between; in 1988 Robert Messer extended this to a full set of "poor man's blackboard bold" macros, overtyping each capital letter with carefully placed I characters or vertical lines.
Not all mathematical authors were satisfied with such workarounds. The American Mathematical Society created a simple chalk-style blackboard bold typeface in 1985 to go with the AMS-TeX package created by Michael Spivak, accessed using the \Bbb command ; in 1990, the AMS released an update with a new inline-style blackboard bold font intended to better match the Times New Roman font. Since then, a variety of other blackboard bold typefaces have been created, some following the style of traditional inline typefaces and others closer in form to letters drawn with chalk.
Unicode included the most common blackboard bold letters among the "Letterlike Symbols" in version 1.0, inherited from the Xerox Character Code Standard. Later versions of Unicode extended this set to all uppercase and lowercase Latin letters and a variety of other symbols, among the "Mathematical Alphanumeric Symbols".
In professionally typeset books, publishers and authors have gradually adopted blackboard bold, and its use is now commonplace, but some still use ordinary bold symbols. Some authors use blackboard bold letters on the blackboard or in manuscripts, but prefer an ordinary bold typeface in print; for example, Jean-Pierre Serre has used blackboard bold in lectures, but has consistently used ordinary bold for the same symbols in his published works. The Chicago Manual of Styles recommendation has evolved over time: In 1993, for the 14th edition, it advised that "blackboard bold should be confined to the classroom" ; In 2003, for the 15th edition, it stated that "open-faced symbols are reserved for familiar systems of numbers". The international standard ISO 80000-2:2019 lists as the symbol for the real numbers but notes "the symbols and are also used", and similarly for,,,, and .

Encoding

, the standard typesetting system for mathematical texts, does not contain direct support for blackboard bold symbols, but the American Mathematical Society distributes the AMSFonts collection, loaded from the amssymb package, which includes a blackboard bold typeface for uppercase Latin letters accessed using \mathbb are encoded in the Basic Multilingual Plane in the Letterlike Symbols area, named DOUBLE-STRUCK CAPITAL C etc. The rest, however, are encoded outside the BMP, in Mathematical Alphanumeric Symbols, specifically from 1D538–1D550, 1D552–1D56B, and 1D7D8–1D7E1. Blackboard bold Arabic letters are encoded in Arabic Mathematical Alphabetic Symbols, specifically 1EEA1–1EEBB.

Usage

The following table shows all available Unicode blackboard bold characters.
The first column shows the letter as typically rendered by the LaTeX markup system. The second column shows the Unicode code point. The third column shows the Unicode symbol itself. The fourth column describes some typical usage in mathematical texts. Some of the symbols are nearly universal in their interpretation, while others are more varied in use.
In addition, a blackboard-bold μ_n is sometimes used by number theorists and algebraic geometers to designate the group scheme of roots of unity.