Versions of the Unix system were determined by editions of its user manuals; for example, "Fifth Edition UNIX" and "UNIX Version 5" have both been used to designate the same version. The Bell Labs developers did not think in terms of "releases" of the operating system, instead using a model of continuous development, and sometimes distributing tapes with patches (without AT&T lawyers' approval). Development expanded, adding the concept of pipes, which led to the development of a more modular code base, and quicker development cycles. Version 5, and especially Version 6, led to a plethora of different Unix versions both inside and outside Bell Labs, including PWB/UNIX and the first commercial Unix, IS/1.