My question is closely related to Glyph substitution: alternate capital Q in URW-Garamond? (LaTeX, Mac OS X) but I am using XeLateX. It looks like there is a hidden mechanism that I do not understand. I had a look at all the glyphs of the font using dp4FontViewer 2.0 and I could not find the Q with the very long swash. Instead, the two Qs shown on the right side of the image below are accessible. Q with code 0051 is used when it is isolated in the main text. It'd be nice if it could replace the one with the long swash which takes place in words with more than a single letter (a single Q then).