Issue: Elsewhere on this web site, there are several mentions of X11 and something called XQuartz. Apple has an X11, but so does XQuartz? Which one is better? Which one am I using? Which one should I be using? Aaarrrgh! Can you explain more about this?

Solution: “The XQuartz Project is an open-source effort to develop a version of the X.org X Window System that runs on Mac OS X. Together with supporting libraries and applications, it forms the X11.app that Apple has shipped with OS X since version 10.5.”

In OS X versions 10.3 (Panther) and 10.4 (Tiger), Apple distributed their own version of X11 based on licensed XFree86 v4.3 and X11R6.6. Since the introduction of OS X 10.5 (Leopard), Apple scrapped developing their own version of X11, and began distributing the XQuartz implementation of X11. For example, as of this writing, OS X 10.7.3 (Lion) utilizes XQuartz X11 2.6.3, OS X 10.6.2 (Snow Leopard) utilizes XQuartz X11 2.3.4, and OS X 10.5.8 (the terminal version of Leopard) utilizes XQuartz X11 2.1.6.

So, if you are using Apple's X11, then you are also using XQuartz's X11. Typically, however, the Apple-distributed version lags behind the XQuartz-distributed version. For example, as of this writing, the current version of XQuartz X11 is 2.7.0, which is somewhat more recent than the version being distributed with OS X 10.7.3. In the past, the Apple-distributed version of XQuartz X11 has, at times, lagged much further behind the XQuartz-distributed version. It is possible to manually install the most recent version of XQuartz X11 by following the instructions on their web page. However, it is important to note that many normal Apple software updates (especially OS X point upgrades, X11 upgrades, and sometimes Security Updates) will overwrite or otherwise disable a manually installed XQuartz distribution of X11. This would necessitate reinstalling the XQuartz X11 after most Apple software updates. Also, the Apple Software Update process does not perform updates for manually installed XQuartz X11, leaving future updates to X11 in the hands of the user.

Early on, the integration of XQuartz X11 within Mac OS X was somewhat buggy and unstable, and often resulted in poor performance (as described here). This made it tempting to install the more up-to-date XQuartz-distributed version, even considering the potential to have to reinstall it after Apple software updates and the need to keep it up-to-date manually. That situation has now improved, and the Apple-distributed version of XQuartz X11 that is provided with OS X performs quite well (modulo a few new wrinkles following the introduction of Snow Leopard).

Personally, I use the Apple standard distribution of XQuartz X11 and do not manually install the latest version from XQuartz. The savings in my time to not have to install, reinstall, and manually update X11 on multiple computers easily wins in the trade-off with having a version of X11 that lags somewhat behind the most current available version. The choice is yours...

Thanks to R. Bontekoe for prompting this discussion.

UPDATE (02/17/12): The developer preview version of OS X 10.8 (Mountain Lion) indicates that the X11 application will no longer be included by default, nor will Apple continue to develop their own dedicated X11 application (which has been based on the XQuartz X11). Instead, Apple will direct users to install the supported XQuartz X11. The first time X11 is needed by Mountain Lion (e.g., when you first try to start an X11-based application), you will be prompted to visit an Apple Knowledge Base article describing how to install the XQuartz X11. It is unclear (but unlikely) that Mountain Lion will subsequently keep track of updating the XQuartz X11, although I suppose that it is not impossible that XQuartz could bundle their X11 as an application installed by the Mac App Store (which would be somewhat more convenient than having to install and upgrade it manually). See MacRumors for more information.

OS Version Compatibility: Mountain Lion, Lion, Snow Leopard, Leopard

Update Status: 17 February 2012