The Primary feature in Double Hung Windows is that both window sashes operate vertically, as compared to Single Hung Windows, where the bottom sash is the only operational section. The appearance of the Double Hung Window is stacked, so that both sashes can slide past each other. The name "Double Hung" comes from the design of both sashes being hung from ropes that are counter-balanced with weights. The glass openings in the upper and lower sash are typically of equal size. The ability to open both sashes creates cross ventilation, where the warm air exits through the top opening and the cooler air enters through the bottom opening.