Windows on Windows action, commonly referred to as WOW or WoW, is a critical component for backward compatibility of legacy code in Windows NT-based operating systems. WoW supports applications using the Win16 API and relies on NTVDM (NT Virtual DOS machine).
Win16 applications run as-is without preemptive multitasking or long filename support. These programs do not take advantage of the 32-bit version of Windows NT.
64-bit versions of Windows do not include the 16-bit WoW subsystem or NTVDM and therefore cannot run 16-bit Windows or DOS applications.