What is a double hung windows 

Unlike single hung windows — where only the bottom sash can be moved — double hung windows feature operable top and bottom sashes. This configuration allows for exceptional ventilation, since you can have cool air blowing through the bottom opening while warm air blows through the top. And, if you want to install an in-window air conditioner, double hung windows allow you to easily do so. Plus, because both sashes can be tilted inward, double hung windows are a smart option for homes with multiple stories, since it could otherwise be difficult to clean the exterior of an inoperable top-floor window.