WinFrame was a multi-user version of Winows NT 3.51 which was fully repackaged by Citrix Systems. At this stage of the product development Citrix Systems licensed the Windows NT 3.51 base operating system from Microsoft. The core development that Citrix delivered was the MultiWin engine. This allowed multiple users to logon and execute applications on a WinFrame server. Citrix was to later license the MultiWin technology to Microsoft, forming the basis of Microsoft's Terminal Services.