How get \Box like symbol with XeLaTeX

Question

The source below, when processed with pdflatex produces the math symbol shown here:

But if I process the source with xelatex, I get error:

./unitsquare.tex:26: LaTeX Error: Command \Box not provided in base LaTeX2e.

Question: How can I get that symbol with xelatex with the fonts in use for that?

\documentclass{article}

\usepackage[T2A,T1]{fontenc}
\usepackage{amsmath}    % avoid \dddot clash
\usepackage{mathtools}  % avoid unicode-math clash
\usepackage{amsthm} % avoid openbox clash

\usepackage{iftex}
\ifTUTeX
  \usepackage{fontspec}
  \usepackage{unicode-math}
  \setmainfont{TeX Gyre Termes}[Scale=1.0]
  \defaultfontfeatures{Scale=MatchLowercase, Ligatures=TeX}
  \setmathfont{TeX Gyre Termes Math}
\else
  \usepackage{newtxtext}  
  \usepackage{newtxmath}
\fi

\usepackage{scalerel}

\newcommand{\unitsq}{\mathord{\scalerel*{\Box}{X}}}

\begin{document}

The unit square is denoted by $\unitsq$.

\end{document}

Related: How to type in these "circle" and "square"?

Answer 1

You might have the impression that you can prepare a document that compiles with both pdflatex and xelatex. This is not true, I'm afraid.

Anyway, the symbol you're probably looking for is \lgwhtsquare corresponding to U+2B1C WHITE LARGE SQUARE, that's unfortunately unavailable with TeX Gyre Termes Math or \mdlgwhtsquare, corresponding to U+25A1 WHITE SQUARE.

If you want to scale the symbol to the height of a capital letter, you can use graphicx (or scalerel, if you prefer).

\documentclass{article}

\usepackage{mathtools}
\usepackage{amsthm}
\usepackage{graphicx}

\usepackage{fontspec}
\usepackage{unicode-math}
\setmainfont{TeX Gyre Termes}
\setmathfont{TeX Gyre Termes Math}

\newcommand{\unitsq}{\text{\resizebox{!}{\fontcharht\font`X}{$\mdlgwhtsquare$}}}

\begin{document}

The unit square is denoted by $\unitsq$.

\end{document}

An attempt to get a “universal” document:

\documentclass{article}
\usepackage{iftex}

\usepackage{mathtools}
\usepackage{amsthm}
\usepackage{graphicx}

\iftutex
  \usepackage{fontspec}
  \usepackage{unicode-math}
  \setmainfont{TeX Gyre Termes}
  \setmathfont{TeX Gyre Termes Math}
\else
  \usepackage[T1]{fontenc}
  \usepackage{newtxtext,newtxmath}
\fi

\iftutex
  \newcommand{\unitsq}{\text{\resizebox{!}{\fontcharht\font`X}{$\mdlgwhtsquare$}}}
\else
  \newcommand{\unitsq}{\text{\resizebox{!}{\fontcharht\font`X}{$\Box$}}}
\fi

\begin{document}

The unit square is denoted by $\unitsq$.

\end{document}

With pdflatex I get

Why does LaTeX balk to \Box? Because you need amssymb for it unless a font package such as newtxmath provides by itself the AMS symbols. And no, unicode-math doesn't provide \Box. Maybe it should…

Answer 2

If you would rather have a larger square than TeX Gyre Termes Math has, you can borrow one from another math font.

\documentclass{article}
\usepackage{unicode-math}

\setmainfont{TeX Gyre Termes}
\setmathfont{TeX Gyre Termes Math}
\setmathfont{STIX Two Math}[
  range=\lgwhtsquare,
  Scale=MatchUppercase ]
\newcommand\unitsq{\lgwhtsquare}

\begin{document}

The unit square is denoted by $\unitsq$.

\end{document}

There are numerous white squares in unicode-math, including \lgwhtsquare, \mdlgwhtsquare, \mdwhtsquare, \mdsmwhtsquare and \smwhtsquare. If none of these are the exact size you want, you can even load any of them with a tweaked Scale= option.