blackboard Bold

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, A, B, C, D, E, .... 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, R^n for Euclidean n-space, and Z for the integers.

C | Q | R | set | Z