In a single hung window, what is the characteristic of the upper sash?

In a single hung window, the defining characteristic is that the upper sash is fixed in place and does not move. This design allows for the lower sash to slide vertically up and down, providing ventilation while maintaining the structural integrity and aesthetic of the window.

In contrast, other window types such as double hung windows allow both sashes to move, which is not applicable here since the question specifically refers to a single hung window. Similarly, the movements described in other options, like sideways movement or tilting of the upper sash, do not occur in this type of window because the upper sash remains stationary. This fixed feature is what distinguishes single hung windows from alternative designs.