Apple's X11 for Mac OS X is available as an optional installation bundled with Mac OS X. To install it when you first install (or upgrade an existing installation of) Mac OS X Panther, you must customize the installation (in the Selection Type phase) and select the X11 checkbox. If you don't install X11 during the Mac OS X installation, you can install it later by inserting the Install Mac OS X Disc 3 CD, then finding and double-clicking the X11User.pkg package in the Packages folder.
The installation places the double-clickable X11 application in the /Applications/Utilities folder. If you're going to build X11-based applications, you'll need to install the X11SDK, which is located as an optional package on the Xcode Tools CD ( /Developer Tools/Packages/X11SDK.pkg ). Instructions for building X11 applications are included in Chapter 8; this chapter simply focuses on using X11.