Bold and italic in math mode in LaTeX
Bold and italic in math mode set symbols in different weights and styles; in LaTeX \mathbf bolds upright letters, \boldsymbol bolds Greek and symbols, and \mathcal gives calligraphic capitals.
LaTeX command
\mathbf{v}Not the symbol you meant? Draw it to find the command →
Letters in math mode are italic by default, which is why single variables already slant. To make a symbol bold and upright, use \mathbf{v}, the usual choice for vectors and matrices. Because \mathbf also removes the italic slant, it suits Latin letters but is not the right tool for lowercase Greek.
To bold a Greek letter or an operator while keeping its italic shape, use \boldsymbol{\alpha} or \boldsymbol{\theta}. This is the standard way to write a bold parameter vector. \boldsymbol comes from amsmath (via amsbsy) and is supported by KaTeX; \mathbf is core LaTeX.
Other alphabets are reached with their own commands: \mathit forces math italic, \mathcal gives calligraphic capitals (used for sets and operators like a Lagrangian), \mathbb gives blackboard bold for number sets, and \mathfrak gives fraktur. \mathcal and \mathbb accept capital letters only.
Other forms
\boldsymbol{\alpha}bold Greek/symbols\mathit{x}math italic\mathcal{L}calligraphicExamples
$\mathbf{A}\mathbf{x} = \mathbf{b}$a bold matrix equation
$\boldsymbol{\theta}$a bold parameter vector in Greek
$\mathcal{L}(\theta)$a calligraphic L for a loss or Lagrangian
$\mathbb{R}^n$blackboard-bold reals with a dimension
$\mathbf{v} \neq \boldsymbol{\nu}$bold Latin v versus bold Greek nu
Tips and common mistakes
\mathbfbolds upright Latin letters, dropping the italic; that is exactly what vectors and matrices want.\mathbfwill not bold lowercase Greek. For a bold Greek letter or symbol, use\boldsymbol{\omega}instead.\mathcaland\mathbbwork on capital letters A to Z only; a lowercase argument will not render as expected.\mathbbneeds amssymb and\boldsymbolneeds amsmath in a full document; both render in KaTeX.- Do not use
\mathitto bold; it only italicises. Combine weight and shape with\boldsymbolwhen you need bold italic.
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Convert your maths to LaTeX →FAQ
How do I make a vector bold in math mode?
Use \mathbf{v}. It sets the letter in upright bold, the conventional style for vectors and matrices. For a bold Greek vector, use \boldsymbol instead.
How do I bold a Greek letter?
Use \boldsymbol{\alpha}. Plain \mathbf does not affect lowercase Greek letters, whereas \boldsymbol bolds them while keeping their usual italic shape.
How do I write a script or calligraphic capital?
Use \mathcal{L}. It produces calligraphic capitals such as those used for a Lagrangian or a family of sets. It accepts capital letters only.
What is the difference between \mathbf and \boldsymbol?
\mathbf makes letters bold and upright (removing italic) and ignores lowercase Greek. \boldsymbol makes symbols bold while preserving their italic shape and works on Greek and operators.