EWMH
The extra rules that let taskbars, pagers and window managers from different projects understand each other under X11.
Extended Window Manager Hints, originally called NetWM, took shape around 2000 when GNOME and KDE developers noticed that ICCCM had nothing to say about things every modern desktop needed. The spec is maintained by freedesktop.org and consists of properties on client windows and the root window, all prefixed _NET_.
With _NET_WM_STATE an application can ask to go fullscreen or maximised, _NET_WM_WINDOW_TYPE tells a dialog from a dock, _NET_WM_NAME finally allowed UTF-8 window titles, and _NET_NUMBER_OF_DESKTOPS standardised virtual desktops. The upshot is that a taskbar from one project works with a window manager from another, and tools like wmctrl can drive almost any of them. Fairly undramatic, which is precisely why it worked.