Classic (environment)

The Classic environment, also known as Classic mode and simply "Classic", is an abstraction layer that allows most legacy applications written for Mac OS 9 and earlier to function under PowerPC versions of Mac OS X. It was supported from early versions of Mac OS X Public Beta through Mac OS X 10.4.11 as a vital part of the transition from classic Mac OS to OS X.

Development
Mac OS X is based on NeXTSTEP, which uses a Unix-based kernel that is inherently incompatible with classic Mac applications. During its early development under the code name Rhapsody, a proof-of-concept called "Blue box" demonstrated that backwards compatibility could be provided through a native application that would run the older software in a sandbox environment. Any problematic legacy program that would normally cause Mac OS 9 to crash could be contained and only require relaunching of the Classic environment without bringing down the main system.

During beta testing of Mac OS X, the minimum version of classic Mac OS to be supported was 9.0.4, which needed to be installed separately. Stability was inconsistent and improved with successive public beta releases. Another transition technology called Carbon was also introduced, which allowed applications which took advantage of the API to run on both Mac OS 9 and PowerPC versions of Mac OS X.

When the first retail version of Mac OS X 10.0 was released in March 2001, the minimum version of classic Mac OS to be supported was 9.1, which was included as part of the installation.

Mac OS X 10.4.11 was the last version to support the Classic environment, which was an optional installation item with Mac OS 9.2.2.

Discontinuation
Mac OS X 10.5, which was part of the Intel transition, dropped support for classic applications entirely. Alternatives for modern versions of macOS include emulators such as Basilisk II, Mini vMac, QEMU, and SheepShaver.