How do I change the math italic font in XeTeX/fontspec?

Question

I have a document where the main font is Gotham Book. So in my preamble I have:

\usepackage{fontspec}
\setmainfont{Gotham Book}

As the fontspec manual describes, this sets the math roman font to also be Gotham Book. But the math italic font remains from the Computer Modern family.

I know that \setmathrm will change the math roman font, and there are cousins \setmathtt, \setmathsf, and \setboldmathrm. But there's no \setmathit.

What am I missing?

Answer 1

If you do not wish to otherwise change your maths symbols, the best solution is to use the mathspec package:

\documentclass{article}
\usepackage{mathspec}
\setallmainfonts(Digits,Latin){Georgia}
\begin{document}
Hello $a+\mathrm{b}=c$
\end{document}

Here, Georgia will be used for the body text and \mathrm, and Georgia Italic will be used for the italic math glyphs.

The unicode-math package (which I kinda wrote) can also do this, but it's somewhat overkill for your purposes and has the additional downside that you need to also load a Unicode mathematics font. Here's an example:

\documentclass{article}
\usepackage{unicode-math}
\setmainfont{Georgia}
\setmathfont{xits-math.otf}
\setmathfont[range=\mathit]{Georgia Italic}
\begin{document}
Hello $a+b=c$
\end{document}

Note that mathspec will not run (yet) on LuaTeX, so if you need a LuaLaTeX solution then you'll need to use unicode-math for now.

Answer 2

Thanks to Will and Leo for pointing out the mathspec package. So now I use:

\usepackage{mathspec} %loads fontspec as well
\setmainfont{Gotham Book}
\setmathrm{Gotham Book}
\setmathfont(Digits,Latin){Gotham Book}

This snippet may need optimization, but works.

Answer 3

The Literal Answer

As of 2019, fontspec should find the bold and italic fonts, and load \mathrm, \mathbf and \mathit when you \setmathrm.

If you need additional set-up for \mathit, the ItalicFont and ItalicFeatures options of \setmathrm set up \mathit, just as the Boldfont and BoldFeatures options set up \mathbf.

\documentclass{article}
\usepackage[svgnames]{xcolor}
\usepackage{fontspec}
\usepackage[paperwidth=10cm]{geometry}

\setmainfont{TeX Gyre Schola}
\setmathrm{TeX Gyre Schola}[
  ItalicFeatures = {Color = green}]

\begin{document}
Text \textit{italic} \(\mathrm{math}\) \(\mathit{italic}\)
\end{document}

What You Also Might Have Meant

In practice, someone asking about how to set the \mathit alphabet likely means the alphabet used for $x$.

To set that, you want to load the unicode-math package, set a math font that matches your main font and then, if necessary, add a line such as

\setmathfont[range=it]{Gotham Book Italic}

You can do likewise for up, bfup, bfit and the other math alphabets.

In unicode-math, you would normally use \symit, \symup, etc. for individual letters used as variables, and \mathit, \mathrm, \mathup, etc. for complete words. However, you can give a package option to interpret \mathit as \symit and make migrated code compile correctly.