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

The window type that consists of an upper and a lower sash that slide vertically within the side jambs is the double hung window. This design allows both the upper and lower sashes to be movable, enabling the user to open either section for ventilation. This feature is particularly advantageous for promoting airflow, as the lower sash can be opened to let in fresh air while the upper sash allows warmer air to escape. The double hung style is prevalent in many home designs due to its functional versatility and classic aesthetic.

Single hung windows, in contrast, only allow the lower sash to move, while the upper sash remains fixed. Casement windows operate differently altogether, swinging outward from hinges on the side, and picture windows are fixed and do not open at all. Therefore, the unique vertical sliding capability of the double hung window distinguishes it as the correct answer.