What mode is key to backward compatibility of Windows 32 bit environment?

The key mode for backward compatibility in a Windows 32-bit environment is the Windows on Windows 64 (WoW64) subsystem. WoW64 allows 32-bit applications to run seamlessly on 64-bit versions of Windows by providing a compatibility layer that translates 32-bit calls to 64-bit operations. This ensures that legacy 32-bit applications can function without modification on modern 64-bit Windows systems.