How are those not simply child processes? I don't understand your use of the word 'threads' here.

Does the Unix world not distinguish between threads and processes? In Win32, threads exist within processes, and you can create new threads or child processes.

The Unix model was invented over a decade before the idea of multithreading percolated into mainstream operating systems at all.

The reason that Windows NT started as it did, was that OS/2 had come out in 1987, with kernel threads, and the idea of multithreading had taken root. SunOS 5 gained threading, too.

Windows NT applications development began with threading available as a mechanism from the start, and with a lot of people in the IBM/Microsoft world already knowing about its use in applications development from OS/2.

Whereas with the Unices it came in more gradually, as the applications had often already been designed. The whole libthread versus libpthread thing made things interesting on SunOS for a few years, too. As did the first attempt (LinuxThreads) at providing threads on Linux.

Which is why I took the effort to explicitly refer to Windows NT on my comment, already expecting some traditional answers from UNIX folks.

Also due to historical reasons POSIX threads are the outcome of every UNIX going their own way implementing threads, finally coming to an agreement years later, with all the plus and minus of relying in POSIX for portable code.