a section, represents a vertical plane cut through an object, in the same way that a floor plan is a horizontal section viewed from above. In the section view, everything cut by the section plane is shown as a bold line, often with a solid fill to show objects cut through, while anything seen beyond is generally shown with a thinner line. Sections are used to describe the relationship between different levels of a building.

In the Observatorium drawing illustrated here, the section shows the outer dome, a second dome visible only inside