What 𝕖 means
It marks a mathematical letter style; the formula supplies its role.
The double-struck e belongs to its own math alphabet. In a formula, letter style helps keep symbols distinct; read the author's definition to see what object it names. Capital blackboard-bold letters are familiar as labels for number sets.
Fonts may draw this character in serifed or simpler forms, but Unicode's design guidance keeps doubled strokes as the defining feature.
A chalkboard workaround became a math font
On blackboards, mathematicians doubled selected strokes to make bold symbols stand out when chalk could not imitate the heavy print face. Typewriters offered a related trick: strike a character twice with a small offset. A 1954 manual on mathematical printing prescribed wavy underlining to mark boldface in manuscripts. Gunning and Rossi's 1965 book on complex analysis is an early printed example of blackboard-bold notation. The chalkboard mark became a typeset math style.
In a 2003 account, Lee Rudolph reported that Robert Gunning traced his own use to seminars at Princeton in the early 1960s, where he learned the notation and passed it to students.
Part of the collection
Don't confuse it with
1
Questions people ask
1Why does 𝕖 show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
More like this
11
How to type or find 𝕖
- Also called
- blackboard-bold e · open-face e
- Search it as
- blackboard bold e · double-struck e · mathbb e · bbold e
- Shows up in
- Anything that takes text · its shape is whatever your font draws
𝕖\1D556𝕖 in HTML source, and the browser displays the character.\mathbb{e} with a math font that includes lowercase double-struck letters; the bbold font family includes them.Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.