CreateMDIWindow

function CreateMDIWindow(

  lpClassName: LPCTSTR;

  lpWindowName: LPCTSTR;

  dwStyle: DWORD;

  X: Integer;

  Y: Integer;

  nWidth: Integer;

  nHeight: Integer;

  hWndParent: HWND;

  hInstance: HINST;

  lParam: LPARAM

):HWND;