The early versions of Windows NT transitioned from the Program Manager of Windows 3.1 as Windows 95 was developed, leading to the integration of the Windows 95 shell into Windows NT. The Windows NT team, while preparing for their launch, considered how to incorporate features from Windows 95, resulting in Windows NT 4.0. The teams maintained communication, with the NT team integrating the Windows 95 user interface and window management capabilities into their system. The integration involved using Windows 95 as a reference for reimplementing features rather than merging code directly. Explorer was integrated into Windows NT's architecture, allowing updates in Windows 95 to avoid redundant changes. To prevent bugs from affecting Windows 95, the NT team enclosed new code within #ifdef WINNT directives. They faced challenges with the sizeof operator due to Windows NT's Unicode awareness, leading to a macro definition for tracking verified instances. Microsoft used an internal source code management system called SLM, which lacked branch support, making the transfer of changes between Windows 95 and Windows NT a manual process.