Ruling gap ratios are an affine-invariant characterization of parallel ruling configurations in scanned documents. This report quantifies the advantage of simultaneous extraction of horizontal and vertical rulings. It demonstrates that every ruling gap ratio can be derived from a minimal set of basis ratios. The effect on the basis ratios of noise on the radial coordinates of individual rulings is analyzed and the dependence of basis-ratio variability on random-phase sampling noise is determined as a function of the spatial sampling rate. The analysis provides insight into already-presented small-scale experimental results on form classification and guidance for future work that requires the extraction of parallel lines from scanned or photographed images.