X11, the "old" [0] Linux graphics server.

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[0] "old" because there's a new kid in town: wayland.

I’ve been reading about Wayland since around 2010 and it still isn’t ubiquitous, and still doesn’t work everywhere.

How long can we call something that’s at least 15 years old “the new kid”?

Wayland has arrived in lot of distros as default graphics server pretty recently, that's why it is OK to be called the new kid, IMO.

And a middle-age'd man named XFree86 lol.