XQuartz is an open-source version of the X.Org X server, a display server for the X Window System that runs on macOS. In 2012, it formally replaced Apple's internal X11 app for OS X Mountain Lion. The name "XQuartz" derives from Quartz, part of the macOS Core Graphics framework, to which XQuartz connects these applications. XQuartz allows cross-platform applications using X11 for the GUI to run on macOS, many of which are not specifically designed for macOS. This includes numerous scientific and academic software projects.