What type of window can move past each other in a vertical plane?

The type of window that can move past each other in a vertical plane is a pivoting window. This design allows one or more sections of the window to rotate around a pivot point, typically located along the side of the frame. As a result, the window can open by swinging inwards or outwards, offering flexibility in ventilation and ease of cleaning.

In contrast, horizontal sliding windows operate on a track and slide from side to side rather than moving vertically. Projecting windows, often referred to as casement windows, swing open but pivot from the side rather than moving past another sash. Awning windows are hinged at the top and open outward, creating an awning effect, but do not have a mechanism for vertical movement past another section. Thus, pivoting windows are uniquely designed to move past one another in a vertical motion, making this choice the correct answer.