The criterion here is simple: all of the fabrics must be different. Not one square is to be the same as another. I can't exactly remember why I decided that, I think that I just wanted it to look really home-made. And the nice thing is that I have asked friends for some of their fabrics, so a bit of them is now in there too. I just need five more squares and I'm ready to go.
Above: my first ever patchwork effort: a cushion that I made when I was a child. Yes that is indeed Frida and Agnetha in the centre. Below: I found my old book, Supersewing Without a Pattern by Carolyn Trager (New York, 1976) when visiting my parents the other day. On the back page was a reminder of what the patchwork cushion was originally going to be: