# unknown problem in siunitx

I'm trying to use the inch mark (the double prime) in a piece of text; in the past I've gotten away with

4 $''$ \times 6 $''$


but for some reason that doesn't work when that line is an item in an itemized list. I'm now experimenting with the siunitx package, but now there's a new problem. This MOLMWE (More or Less Minimal Working Example)

\documentclass{article}
\usepackage{siunitx}
\usepackage[LY1]{fontenc}
\usepackage{fontspec}
\usepackage{xltxtra}

\setromanfont[Numbers=OldStyle]{Linux Libertine O}

\sisetup{detect-all = true}
\DeclareSIUnit[number-unit-product = {}]{\inchQ}{\prime\prime}

\begin{document}

\begin{itemize}

\item \SI{4 \times 6}{\inchQ} index cards

\end{itemize}

\end{document}


produces an incorrect result; I'd attach an image, but I don't have sufficient reputation for that privilege. What's the problem, if you can spot it from just the code, and how do I fix it?

P.S. As for the MOLMWE: I included possibly extraneous material because I wasn't sure whether it was relevant and decided to play it safe.

• siunitx already defines the unit \arcsecond (and \arcminute). – Qrrbrbirlbel Aug 27 '13 at 20:55
• The error is "invalid-token-in-number", caused by \times. – egreg Aug 27 '13 at 20:59
• Does that mean I shouldn't be using \times in text? And if so, is there a better way to incorporate that symbol? – user35810 Aug 27 '13 at 21:01
• @user35810 siunitx converts x in the input as an product (see the options input-product and output-product). – Qrrbrbirlbel Aug 27 '13 at 21:03
• What's wrong with just doing $4''\times6''$? If you want to use the text font instead of the maths font, then just try $\rm 4''\times6''$. – Thruston Aug 27 '13 at 21:04

I’d use the UTF characters here and not the \prime\prime (better would be siunitx’ default setting {}^{\prime\prime} which is '', anyway). That way, the glyphs are taken from the same font as the digits.

## Code

\documentclass{article}
\usepackage{fontspec}
\usepackage{siunitx}
\setromanfont[Numbers=OldStyle]{Linux Libertine O}
\sisetup{
detect-all = true,
text-arcsecond=″,
text-arcminute=′,
math-arcsecond=″,
math-arcminute=′,
}
\DeclareSIUnit[number-unit-product={}]\inchQ{\SIUnitSymbolArcsecond}
\DeclareSIUnit[number-unit-product={}]\footQ{\SIUnitSymbolArcminute}
\begin{document}
\begin{itemize}
\item Arcseconds
\begin{itemize}
\item \SI{4 x 6}{\arcsecond}
\item $\SI{4 x 6}{\arcsecond}$
\end{itemize}
\item Inches
\begin{itemize}
\item \SI{4 x 6}{\inchQ}
\item $\SI{4 x 6}{\inchQ}$
\end{itemize}
\end{itemize}
\end{document}


## Output

• Extra marks for maintaining the semantic difference between an arc and inches :-) – Joseph Wright Aug 27 '13 at 21:14