A new ''lost-area'' cross-section construction and validation technique is based on the area balance of a graben system formed above a detachment. Area balance requires that the area displaced below the original regional level of each bed in a graben (the lost area) be balanced by the displaced area at the boundary of the system, given by the product of the displacement times the depth to detachment. This relationship is a straight line on a plot of lost area versus depth. The slope of the line is the displacement, and the depth intercept is the depth of the detachment. In general, beds within the graben system undergo both visible and homogeneous layer-parallel extension. The homogeneous component can be determined from the width of the graben system, the lost area, and the depth of detachment. Conversely, the detachment depth may be calculated from the total layer-parallel extension and the lost area. The techniques are illustrated by application to models and to very well constrained natural examples, an earthquake-induced slope failure in Alaska and kilometric-sized horsts and grabens in the Black Warrior foreland basin of Alabama. A half graben without footwall uplift and a full graben are the same on a lost-area diagram. A half graben with footwall uplift linked to the hanging wall extension requires that both the hanging wall and footwall areas be included to determine the detachment depth and displacement. All the models show layer-parallel extension e in the upper levels that are less than the crustal-scale strain &bgr;.