I don't know English, well. So please understand me even if statements are wrong.

I saw following statement from wglMakeCurrent func page on MSDN(http://msdn.microsoft.com/ko-KR/libr.../dd374387.aspx).

Handle to an OpenGL rendering context that the function sets as the calling thread's rendering context.
If hglrc is NULL, the function makes the calling thread's current rendering context no longer current, and releases the device context that is used by the rendering context. In this case, hdc is ignored.
So, I thought that wglMakeCurrent(NULL, NULL) calls ReleaseDC() inner code of it. Although it's not ReleaseDC(), Device Context will be released, I think.
However, almost sample codes on online call ReleaseDC() after wglMakeCurrent(NULL, NULL). Even NeHe's sample code is also, too.

What does that mean?