Windows on Windows
Windows 9x operating systems, which were made on top of DOS, had both 16 and 32-bit systems, so if it kept those 16-bit parts, the OS could run 16-bit software without emulation. NT-based versions are very different in how they run, so they need a more sophisticated solution. There are two main techniques to let 16-bit programs run on 32-bit versions of Windows (with some limitations) called thunking and shimming.