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 ${\displaystyle \mathbb {N} }$ (natural numbers), ${\displaystyle \mathbb {Z} }$ (integers), ${\displaystyle \mathbb {Q} }$ (rational numbers), ${\displaystyle \mathbb {R} }$ (real numbers), and ${\displaystyle \mathbb {C} }$ (complex numbers).

To imitate a bold typeface on a typewriter, a character can be typed over itself (called double-striking);^[1] symbols thus produced are called double-struck, and this name is sometimes adopted for blackboard bold symbols,^[2] for instance in Unicode glyph names.

In typography, a typeface with characters that are not solid is called inline, handtooled, or open face.^[3]