𝒅 Mathematical Bold Italic Small D

What 𝒅 means

It has no fixed meaning outside its formula.

The extra weight and slant mark a separate mathematical symbol. Some scientific styles use bold italic for vectors—quantities with size and direction—or matrices, rectangular arrays of values. Others assign the type styles differently, so follow the author's notation key.

Where did mathematical boldface come from?

It combines older printing cues that grew from mathematical practice.

René Descartes used lowercase italic letters for unknowns in 1637. Vector writers later wanted a way to distinguish vectors—quantities with size and direction—from scalars, quantities with size but no direction. James Clerk Maxwell promoted German blackletter (Fraktur) for vectors in 1873; Oliver Heaviside described using bold Clarendon type, a heavy serif face, in 1886.

The digital math alphabet had a separate purpose: a 2000 Unicode proposal developed with the STIX Project, a publisher-backed scientific-font effort, aimed to give formulas a shared character repertoire for exchange and archiving.

Technical and precise: in formulas, weight and slant are part of the notation.

Don't assume 𝒅 denotes a vector; mathematical styles differ.

Part of the collection

𝐁

Bold Letters (Bold Text)

Don't confuse it with

2

𝐝

Mathematical Bold Small D

For bold italic d vs bold d, notice the slant: 𝐝 is upright.

𝑑

Mathematical Italic Small D

For bold italic d vs italic d, compare the weight: 𝑑 slants but is not bold.

Related characters

1

d

Latin Small Letter D

This is the ordinary Latin small d in Unicode's compatibility mapping for 𝒅. A formula can still use the plain and styled forms for different quantities.

Questions people ask

2

Why is 𝒅 showing as an empty box?

It can happen when the selected font lacks this mathematical glyph. Switch to a math font with broad Unicode coverage; the underlying text can still contain 𝒅.

Why does 𝒅 turn into a plain d?

It can happen during compatibility normalization. Unicode's NFKC process folds this mathematical form to ordinary Latin d, removing the visual distinction. Math markup preserves styling as notation instead of changing the character.