• A simple rectangular face has one contour and four edges.

• A contour traverses a boundary so that the part face is always on the right-hand side (RHS). For an external contour the direction of traversal is clockwise. For an internal contour the direction of traversal is counterclockwise.

• If a part is extruded from a sketch that has a U-shaped cross section there will be separate surfaces at each leg of the U-channel.

• If a part is extruded from a sketch that has a square-shaped cross section, and a slot feature is then cut into the part to make it look like a U-channel, there will be one surface across the legs of the U-channel. The original surface of the part is represented as one surface with a cut through it.