How to Install X11 in OS X
This guide explains how to install X11 in OS X. However, X11 is no longer a part of macOS (since OS X 10.8 Mountain Lion). Apple now suggests users to switch to XQuartz project (more info here).
Starting with OS X 10.5 (Leopard) X11 came installed by default. Unless you went out of your way not to install it during the Leopard installation, you’ll find it in /usr/X11
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