Double-Struck Numbers — Outlined Digits

Double-struck numbers — also called blackboard bold digits — have a distinctive hollow, outlined appearance. Originally used in mathematics to denote special number sets, they're now popular for aesthetic text formatting.

These digits (𝟘-𝟡) pair beautifully with double-struck letters (𝔸-ℤ) for a consistent outlined style. The combination is popular for academic aesthetics, minimalist profiles, and creative formatting.

Double-struck digits are part of Unicode's Mathematical Alphanumeric Symbols block and have good cross-platform support on modern devices.

The style has a precise mathematical origin. 'Blackboard bold' was invented by lecturers who couldn't make chalk write in true bold, so they doubled the vertical strokes of a letter to signal boldface by hand. It became the standard way to write the great number sets: ℕ (naturals), ℤ (integers), ℚ (rationals), ℝ (reals) and ℂ (complex numbers). The digits 𝟘–𝟡 were added so the numerals could match that famous lettering — which is why they read as 'academic' or 'mathematical' rather than merely decorative.

One quirk of the set: because these characters exist to serve mathematics, only the ten digits and the letters were encoded — there is no double-struck comma, currency sign or percent. If you write a hollow-outline price or date, the digits will render but the separators fall back to ordinary characters, so 𝟙,𝟘𝟘𝟘 mixes an outlined thousand with a normal comma. It's usually invisible at a glance, but worth knowing before you build a whole styled number out of them.