A single-hung window, also known as a single-sash window, is a type of window with a lower, operable sash and an upper, fixed sash. This means that the bottom part of the window slides up and down, while the top part remains stationary. Single-hung windows provide ventilation through the bottom sash.

A double-hung window, also commonly referred to as a double-sash window, is a type of window that has two operable sashes that slide up and down. There are two main operable parts of a double-hung window — the top sash and the bottom sash. They are both able to open to provide ventilation. Because heat rises, warm air is able to escape out of the top sash, while cool air from the outside is able to flow through the lower sash opening to help provide increased comfort. This makes them an excellent choice for two-story homes.