Mac OS
To run classic Mac OS applications, Mac OS X Server 1.0 includes the "Blue Box", which essentially ran a copy of Mac OS 8.5.1 (this could be updated to Mac OS 8.6 in version 1.2 and later) in a separate process as an emulation layer. Blue Box would eventually be renamed as the "Classic Environment" in Mac OS X, featuring the latest version of Mac OS 9.