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X Window System - Wikipedia
Getting X - X Window System
The X New Developer’s Guide: X Window System Concepts
An introduction to X by The Linux Information Project (LINFO)
What is the X Window System? (X, Xorg, X11) - ByteXD
What Is X11? | Baeldung on Linux
An Introduction to the X Window System - O'Reilly Media
X Window System protocols and architecture - Wikipedia
Documentation for the X Window System