What is the + in GTK+? [GTK 2.x]

Peter Mattis informed the gtk mailing list that:

"I originally wrote gtk which included the three libraries, libglib, libgdk and libgtk. It featured a flat widget hierarchy. That is, you couldn't derive a new widget from an existing one. And it contained a more standard callback mechanism instead of the signal mechanism now present in gtk+. The + was added to distinguish between the original version of gtk and the new version. You can think of it as being an enhancement to the original gtk that adds object oriented features."

Although much has changed with GTK+, and Peter, Spencer and Josh don't have any direct involvement these days, the name is kept to keep a link with the heritage of GTK+.

GTK+ is now additionally based upon the Pango and ATK libraries, that provide text layout and rendering and accessibility interfaces.