Why are the generic version numbers called NTDDI?

In my earlier discussion on the variety of symbols that describe the target Windows version, I pointed out that the NTDDI symbols attempt to cut through the mess and consolidate everything into a single symbol. But why the name NTDDI?

One of my colleagues contacted me privately with the story. When setting out to change the operating system version number, my colleague was shocked to find so many different version number mechanisms were scattered throughout the various Windows header files. It so happened that the DDK people were already in the process of cleaning up the version number mess and were using NTDDI as their version number system. Seeing no reason to invent a new different system for user mode, my colleague proposed using the DDK system in the SDK and asked if anybody had any better ideas.

Nobody came up with any better ideas, no compelling reason why we should have two different version number systems, so the NTDDI name stuck. And it stands for NT Device Driver Interface.