Double-hung windows are probably the most common type of window. They open by sliding the bottom half of the window up or sliding the top half down. Because only half of the window area can be open at one time, they provide less ventilation than casement style windows. An advantage of double-hung windows is they can be used to create air movement in the home by opening the top portion of the windows on one side of the house and opening the lower portion of the windows on the coolest side of the house. Air will naturally rise out of the higher opening, pulling cooler air in the lower opening.
Older double-hung windows have weight-and-pulley systems to make them easier to open. Unfortunately, these systems tend to stick or rattle and it is hard to stop air from leaking around the pulley openings. Newer models have spring-tension devices and work far better.