U211D

u211D refers to the Unicode code point U+211D, the character known as the double-struck capital R. Rendered as ℝ, it is commonly used in mathematics to denote the set of real numbers.

Origin and typography: The symbol arises from the practice of blackboard bold notation, where standard symbols were drawn with extra strokes on chalkboards to denote special mathematical sets. In printed text it appears as a double-struck letter, and support for ℝ is provided by most modern fonts that include the Letterlike Symbols block.

Usage: ℝ is widely used across mathematics, especially in analysis, linear algebra, and topology, to denote the real number system. It is part of a family of blackboard bold symbols such as ℕ for natural numbers, ℤ for integers, ℚ for rationals, and ℂ for complex numbers. A related symbol, ℜ (U+211C), denotes the real-part operator in some texts.

Encoding and compatibility: In LaTeX, real numbers are typically written as \mathbb{R} with the amsfonts or amssymb package. In Unicode, U+211D produces ℝ. If a font does not include this character, a plain R may be displayed instead. In HTML, the character can be referenced by numeric entities such as ℝ or ℝ.

See also: Blackboard bold, real numbers, complex numbers, set notation.