Tris Hussey writes (p.181) "...look in the theme's description; if it says that it is designed to be a parent theme..."
He uses the Twenty Ten as his example, but I cannot find the phrase or tag of "parent theme" anywhere in what I would presume is its description.
In addition, why doesn't the Free Themes Directory tag filter include "parent theme"?
How does one discover this info? Are all themes now supposed to accommodate child themes?