featuresets, sometime before June 2000, Gunnar Ritter ported Joy's vi codebase (taken from 2.11BSD, February 1992) to modern Unix-based operating systems, such as Linux and FreeBSD. Initially, his work was technically illegal to distribute without an AT&T source license, but, in January 2002, those licensing rules were relaxed, allowing legal distribution as an open-source project. Ritter continued to make small enhancements to the vi codebase similar to those done by commercial Unix vendors still using Joy's codebase, including changes required by the POSIX.2 standard for vi. His work is available as Traditional Vi, and runs today on a variety of systems. But although Joy's vi was now once again available for BSD Unix, it arrived after the various BSD flavors had committed themselves to nvi, which provides a number of enhancements over traditional vi, and drops some of its legacy features (such as open mode for editing one line at a time). It is in some sense, a strange inversion that BSD Unix, where Joy's vi codebase began, no longer uses it, and the AT&T-derived Unixes, which in the early days lacked Joy's editor, are the ones that now use and maintain modified versions of his code. Impact Over the years since its creation, vi became the de facto standard Unix editor and a hacker favorite outside of MIT until the rise of Emacs after about 1984. The Single UNIX Specification specifies vi, so every conforming system must have it. vi is still widely used by users of the Unix family of operating systems. About half the respondents in a 1991 USENET poll preferred vi. In 1999, Tim O'Reilly, founder of the eponymous computer book publishing company, stated that his company sold more copies of its vi book than its emacs book. Interface vi is a modal editor: it operates in either insert mode (where typed text becomes part of the document) or command mode (where keystrokes are interpreted as commands that control the edit session). For example, typing while in command mode switches the editor to insert mode, but typing again at this point places an "i" character in the document. From insert mode, pressing switches the editor back to command mode. A perceived advantage of vi's separation of text entry and command modes is that both text editing and command operations can be performed without requiring the removal of the user's hands from the home row. As non-modal