What 𝕚 means
Read the letter through the equation around it.
It's a blackboard-bold i: the same Latin letter i rendered with doubled strokes. In mathematical notation, different type styles can keep symbols distinct. In a formula, 𝕚 might be a variable, an index, or a quantity the author defines.
Why are some letters double-struck?
It began as a practical way to mark bold letters by hand.
Chalk and typewriters could not easily reproduce heavy printed type, so writers doubled selected strokes on a blackboard or overstruck typed letters. Accounts place early development in France in the late 1950s and its uptake at Princeton in the early 1960s; mathematical books used the style by the mid-1960s.
That workaround became a printed notation: double-struck capitals commonly mark sets such as the real or complex numbers. The double-struck alphabet also includes lowercase mathematical letters, so 𝕚 can distinguish a symbol in an equation without assigning it one universal meaning.
Part of the collection
Don't confuse it with
1Questions people ask
1Can text tools turn 𝕚 into plain i?
Yes. Compatibility normalization—a way for text systems to standardize forms for matching—can fold mathematical styles back to Latin letters, so 𝕚 becomes i and the distinction disappears.
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How to type or find 𝕚
- Also called
- blackboard-bold i
- Search it as
- blackboard · doublestruck · mathbb · bbold
- Shows up in
- Anything that takes text · its shape is whatever your font draws
𝕚\1D55Abbold in LaTeX, use \mathbb{i}; its font includes lowercase forms. A Unicode math engine can enter 𝕚 directly when the chosen font supports it.Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.