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
- Insert your OS X Tiger Install Disc (#1). Scroll down (it took me forever to realize you had to scroll down) to locate the Optional Installs icon and double-click it
- Click Continue on the first screen
- Read the Software License Agreement and then click Continue
- Click Agree
- Select a location and (you guessed it) click Continue
- Click the small triangle directly next to Applications and then place a check in the box labeled X11. When you’re ready, click Upgrade.
- Enter your password when prompted, and then click OK
- The installation won’t take too long
- After X11 has successfully installed, click Close
- Congrats, you now have a fresh install of X11 in OS X! It can now be found in Applications > Utilities > X11
