Windows 1.0 "BYTE" build is the earliest build of Microsoft Windows confirmed to exist. It was shown in the December 1983 issue of BYTE Magazine, though the build itself is much older and was already obsolete by the time the article was published as Microsoft redesigned the user interface sometime before the official announcement on 10 November 1983, and the new UI had little in common with this early one.
The interface is very similar to Microsoft's DOS applications such as Word and Multiplan which were also released or in development at the time, with a command bar at the bottom of the screen, and featured overlapping windows that were later replaced by tiling windows. It was demonstrated to certain members of the press someday after May 1983. It remains unclear whether the build and applications demonstrated were actually fully functional or just cleverly crafted to create an illusion of a working system.
Photos of various MS-DOS computers running this build were also published.