Mac OS 9 also adds some transitional technologies to help application developers adopt some OS X features before the introduction of the new OS to the public, again easing the transition. These include new APIs for the file system, and the bundling of the Carbon library that apps could link against instead of the traditional API libraries — apps that were adapted to do this can be run natively on OS X as well. Other changes were made beginning with the Mac OS 9.1 update to allow it to be launched in the "Classic Environment" within OS X. This is a compatibility layer in OS X versions prior to 10.5 (in fact an OS X application, originally codenamed the "blue box") that runs a nearly complete Mac OS 9 operating system, allowing applications that have not been ported to Carbon to run on OS X. This is reasonably seamless, though "classic" applications retain their original Mac OS 8/9 appearance and do not gain the OS X "Aqua" appearance.