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).