Mac OS X will bring to the Macintosh platform important new features and enhancements requested by Apple's customers and developers. Apple's goal is to ease your transition to Mac OS X by making as few changes as possible to the programming interface for Mac OS 8. To that end, Apple has created Carbon.
Carbon is a set of programming interfaces you can use to build Mac OS X applications that also run on Mac OS 8 and 9 (versions 8.1 and later). Carbon includes about 70 percent of the existing Mac OS APIs, covering about 95 percent of the functions commonly used by applications. Carbon will allow you to take advantage of many advanced Mac OS X features while preserving your investment in Mac OS 8 and 9 source code. Because Carbon supports the Mac OS 8 and 9 and Mac OS X runtime environments, a single CFM executable will run on both Mac OS 8/9 and Mac OS X.