Child Windows

    
HWND       mainWnd;
HWND       childWnd1;
HWND       childWnd2;

LRESULT CALLBACK MainWndProc(HWND hWnd, UINT uiMsg, WPARAM wParam, LPARAM lParam)
{

   switch(uiMsg)
   {
        case WM_SIZE:
            int width  = LOWORD(lParam);
            int height = HIWORD(lParam);

            SetWindowPos(childWnd1, NULL, 0, 0, width/2, height, 0);
            SetWindowPos(childWnd2, NULL, width/2, 0, width, height, 0); 
         break;
   }

    return DefWindowProc(hWnd, uiMsg, wParam, lParam);
}

int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
{
   // You should replace these and just register the classes

   RegisterMainWindowClass();
   RegisterChildWindowClass();

   // create the main window

   mainWnd = CreateWindow( lpszMainWndClass,
                           "My Game Editor",
                           WS_OVERLAPPEDWINDOW,
                           CW_USEDEFAULT,
                           CW_USEDEFAULT,
                           640,
                           480,
                           NULL,
                           NULL,
                           GetModuleHandle(NULL),
                           NULL );

    if(!mainWnd) return 0;

    // create the child windows

    childWnd1 = CreateWindow( lpszChildWndClass,
                              NULL,
                              WS_CHILD | WS_VISIBLE,
                              0,
                              0,
                              640/2,
                              480,
                              mainWnd,
                              NULL,
                              GetModuleHandle(NULL),
                              NULL );

    // and do the above again for the next window...

    // ....blalbllal

  
    ShowWindow(mainWnd, SW_SHOWNORMAL);
    UpdateWindow(mainWnd);

    // handle messages>

    MSG msg;
    while(GetMessage(&msg, NULL, 0, 0)){  
         TranslateMessage(&msg);
         DispatchMessage(&msg);
    }

    return msg.wParam;
}