I'm using XeLaTeX and EB Garamond to produce a facsimile of a 17th-century French pamphlet. EB Garamond, at least with its default settings, seems to select the long-tailed Q only when the Q is followed by at least three more characters:
\documentclass{article}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{EB Garamond}
\begin{document}
\noindent
Qu'il viue\\
Qui soubmettant\\
Quiconque vienne
\end{document}

However, in the original pamphlet the long-tailed Q is used even for shorter words such as "Qui". How can I get my document to use the long-tailed Q for the word "Qui"?

Qaaadding\mbox{}works, not withQui\mbox{}– egreg Nov 02 '14 at 17:01luatexwe can force the issue by usingfontchar{}(http://tex.stackexchange.com/q/120736/7883), but I don’t usexetexmuch and don’t know if there’s a way to useXeTeXglyphhere, since EB Garamond does not play tricks with the Private Use Area (and it’s good that it doesn’t). – Thérèse Nov 02 '14 at 17:01luatexyou can even write a feature file withsub \Q by \Q.long;so that you don’t need to remember to use the\fontchar{}macro every time Q appears. – Thérèse Nov 02 '14 at 17:05