OpenStep is an object-oriented API specification for an object-oriented operating system that uses any modern operating system as its core, principally developed by NeXT with Sun Microsystems. It is important to recognize that while OpenStep is an API specification, OPENSTEP (all capitalized) is a specific implementation of this OpenStep developed by NeXT. While originally built on a Mach-based Unix (such as the core of NEXTSTEP), versions of OPENSTEP were available for Solaris and Windows NT as well. Furthermore the OPENSTEP libraries (the libraries that shipped with the OPENSTEP operating system) are in fact a superset of the original OpenStep specification.