[Tex/LaTex] unknown problem in siunitx

siunitxxetex

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.

Best Answer

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

enter image description here

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