What ℂ means
ℂ names the set of all complex numbers. A complex number can be written as a + bi: a and b are real numbers, and i is the square root of −1. In ℂ∖ℝ, the symbol means complex numbers without the real numbers.
Blackboard bold arose as a practical way to make bold mathematical letters readable on chalkboards and typewriters; by the late 1950s and early 1960s, mathematicians were using doubled or offset strokes. Older books may use plain C or bold C for the same set. The same convention marks other number sets: ℕ for natural numbers, ℤ for integers, ℚ for rational numbers, and ℝ for real numbers.
For editable formulas, write \mathbb{C} in LaTeX with an AMS maths package, or enter DoubleStruckCapitalC in Wolfram Language.
Formal mathematical notation; it marks a number-set convention rather than casual emphasis.
Plain C can be a variable, so it may not clearly mark the set.
What it means when someone sends it to you
3 readingsDon't confuse it with
2Questions people ask
1Why does ℂ look different in different fonts?
Fonts draw the double-struck strokes differently, so the shape of the C and its extra strokes can vary.
How to type ℂ
ℂ\2102Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.