Which window type consists of an upper and a lower sash that slide vertically in the side jambs?

A double-hung window is characterized by its two sashes that can move independently. The upper sash slides down, while the lower sash slides up, allowing for versatile ventilation options. This design is particularly beneficial in various climates, as it allows the user to control airflow by adjusting the position of each sash. It's a popular choice for residential and commercial buildings due to its classic appearance and functional design.

In contrast, a casement window is hinged at the side and opens outward, primarily offering ventilation from one side. The horizontal-slider window consists of two sashes that slide horizontally rather than vertically, which does not fit the described functionality. An awning window is hinged at the top and swings outward, typically allowing ventilation while protecting the interior from rain; this also does not match the characteristics of the window in question. Thus, the definition and functionality of the double-hung window make it the correct choice.