Letterlike Symbols

ℝ, ℕ, ℤ, ℚ and ℂ: the number sets

Mathematicians write the main sets of numbers in double-struck (or “blackboard bold”) capitals: ℕ natural numbers, ℤ integers (from the German Zahlen), ℚ rational numbers (quotients), ℝ real numbers and ℂ complex numbers. Unicode gives each its own code point in this block, which is why they are missing from the Mathematical Alphanumeric block used for double-struck text. In LaTeX they are \mathbb{R} and friends. Our pages on the real numbers symbol and natural numbers symbol explain the notation in detail.

Letterlike signs in everyday use

• № (numero sign, U+2116) abbreviates “number” in Russian, Bulgarian and many other languages, and appears on French and Spanish documents. See the numero sign.
• ℮ (estimated sign) on European packaging means the quantity meets EU average-fill rules. It is not the letter e.
• ℓ (script small l) is an informal symbol for the litre, used to avoid confusion with the digit 1. The SI symbol is L or l.
• ℞ (prescription take) heads a medical prescription, and ℅ means “care of” on envelopes.
• ℹ (information source) marks an information notice.

When not to use them

Several letterlike symbols exist only for compatibility with older character sets. Unicode recommends ordinary letters instead: write °C with a degree sign plus C rather than ℃, the letter Å rather than the angstrom sign (U+212B), and Greek Ω rather than the ohm sign. They look identical but sort and search differently. The same applies to K for kelvin (U+212A). Browse the unit symbols collection for the recommended forms.