Balanced geological cross sections are an important aid to understanding thrust-belt structures and to estimating their hydrocarbon-bearing potential. Two approaches to section balancing have been taken in the past: construction of retrodeformable sections from raw data, and modification of previous interpretations after an evaluation procedure. These approaches may be unified by use of a Langrangian grid. Projection of datum locations, dips, and stratigraphic thicknesses onto the section plane is performed directly by calculation, or graphically, using plunge lines. Spline interpolation and parallel-fold modeling are the simplest ways of filling gaps in the sectional data, but isogon interpolation yields rheologically more realistic results. Faults are interpolated using cutoff geometries as constraints. Fault tips are located from distance-displacement plots, whereas the depth to detachment is obtained by a modified Chamberlin construction. Interpolations made on the section are directed back to the geologic map via plunge lines. Although these techniques of section construction do not guarantee a balance, especially when complexities such as growth structures, diapirs, or strike-slip faults are present, they eliminate many potential errors.