In architectural terms, a knee wall is the flat plane formed when a sloping surface, typically the roof plane, intersects with the wall at a point that is lower than the normal ceiling height. In other words it's usually a short wall in an attic that connects to the sloping roof. It got it's name because it usually came up to about knee height. Often, they are referred to by their height, for example: "a 6-foot (1.8-meter) knee wall" or "a 5-foot (1.5-meter) knee wall."

Though most often found in attics, knee walls can be built in other places as well. Sometimes people build these short walls to function as room dividers, for example. They're also sometimes installed in entryways as a decorative element.

At least two practical reasons can be given for the existence of knee walls. The first is that they can be part of the building’s structural integrity. To clarify this concept, knee walls can also be bearing walls. Bearing walls help support the weight of the structure above them. They are also called load bearing walls. Attic knee walls can even be added to a building that has already been constructed if the building exhibits structural problems, such as a sagging roof.

The second practical reason has more to do with the comfort and livability of the space in which the knee wall is found. Imagining a finished attic space will help illustrate this. If there were no knee walls in an attic, it would be very difficult to furnish the space in a comfortable manner because there would be tiny angles where the ceiling and wall joined, instead of a flat wall surface to furnish against. The attic space would have a strange feel and scale to it. Knee walls improve the livability of finished attic spaces, creating a welcoming and relaxing room.

In addition, the space formed by the presence of a knee wall often makes great storage space. Often, there is quite a bit of room between the short wall and the vertex formed by the connection between the floor and the ceiling. Building a knee wall door or hatch into the face of the knee wall can access this space. Making access to knee wall space for storage is a good option to consider if an attic is being finished, but the building owner does not want to entirely give up the storage opportunity that an attic typically provides.