The “LW” window was developed to realize a bright, pleasant and fulfilling home environment. In addition to high heat insulating performance, LW’s design and structure help integrate the inner and outer environments. The window is designed so that all frames are hidden from view from the inside when the window is either closed or open. Its lock handle can be tucked away inside the vertical frame and the window can be opened fully. These features create a pleasant sense of oneness of the outer and inner space when the window is opened, and of connectedness when the window is closed.