The history of the MS C runtime DLL

In the earliest days, there was the Win32 SDK shipped with the NT betas and the final release of NT 3.1. The CRT DLL was called CRTDLL.DLL.

Visual C++ 1.0 for NT shipped around the time of NT 3.1 release, and it used MSVCRT10.DLL. This was followed by MSVCRT20.DLL (for 2.x) and MSVCRT40.DLL (for 4.0).

Visual C++ 4.2 introduced the now famous MSVCRT.DLL, which was also used by 5.0 and 6.0. The 6.0 MSVCRT had a new heap allocator that exposed bugs in existing apps, forcing MS to issue the Microsoft Libraries Update.

As a result, starting with Win2000 the MSVCRT.DLL was now part of Windows. Future versions of Visual C++ used MSVCR70.DLL etc. For 7.x the DLLs was supposed to go into the application directory. 8.0 and 9.0 used SxS (with the exception of Win2000 and older where it was supposed to be placed in System32, if I remembered correctly). 10.0 abandoned SxS and always used System32, This is also true for 11.0 and 12.0.

14.0 will split the CRT into two parts, one is the version specific vcruntime140.dll etc, and the other is the non version specific backward compatible appcrt.dll and desktopcrt.dll. See MS's blog article for more details.