About 6,100,000 results
X Window System - Wikipedia
The X New Developer’s Guide: X Window System Concepts
X.Org
An introduction to X by The Linux Information Project (LINFO)
X Window System protocols and architecture - Wikipedia
What Is X11? | Baeldung on Linux
GettingStarted - X Window System
An Introduction to the X Window System - O'Reilly Media
What is the X Window System? (X, Xorg, X11) - ByteXD