of a building. In the Observatorium drawing illustrated here, the section shows the dome which can be seen from the outside, a second dome that can only be seen inside the building, and the way the space between the two accommodates a large astronomical telescope: relationships that would be difficult to understand from plans alone. A sectional elevation is a combination of a cross section, with elevations of other parts of the building seen beyond the section plane. Geometrically, a cross section is a horizontal orthographic projection of a building on to a vertical plane, with the vertical plane cutting through the building. Isometric and axonometric projections Isometric and axonometric projections are a simple way of representing a three dimensional object, keeping the elements to scale and showing the relationship between several sides of the same object, so that the complexities of a shape can be clearly understood. There is some confusion over the distinction between the terms isometric and axonometric. "Axonometric is a word that has been used by architects for hundreds of years. Engineers use the word axonometric as a generic term to include isometric, diametric and trimetric drawings." This article uses the terms in the architecture-specific sense. Despite fairly complex geometrical explanations, for the purposes of practical drafting the difference between isometric and axonometric is simple (see diagram above). In both, the plan is drawn on a skewed or rotated grid, and the verticals are projected vertically on the page. All lines are drawn to scale so that relationships between elements are accurate. In many cases a different scale is required for different axes, and again this can be calculated but in practice was often simply estimated by eye. An isometric uses a plan grid at 30 degrees from the horizontal in both directions, which distorts the plan shape. Isometric graph paper can be used to construct this kind of drawing. This view is useful to explain construction details (e.g. three dimensional joints in joinery). The isometric was the standard view until the mid twentieth century, remaining popular until the 1970s, especially for textbook diagrams and illustrations. Cabinet projection is similar, but only one axis is skewed, the others being horizontal and vertical. Originally used in cabinet making, the advantage is that a principal side (e.g. a cabinet front) is displayed without distortion, so only the less important sides are skewed. The lines leading away from the