The difference in time required by a signal to propagate through conductors in the same cable due to differences in the physical lengths of the pairs caused by different twist ratios. For example, each of the four pairs in a Cat 5 or Cat 5e cable has a slightly different twist ratio, in order to minimize crosstalk. The difference in twist ratios results in a slightly different lay length, i.e., physical length if the cable were to be untwisted and laid flat, for each pair.This causes built-in propagation delay skew simply because it takes more time for a signal to travel a longer physical path, as illustrated in Figure D-2. As too much delay skew will cause transmission errors because of timing differences between the signals spread across the various pairs, some cable manufacturers take advantage of the skin effect and use foamed insulation, rather than solid insulation, on the conductors. Other manufacturers simply stagger the twists. In fiber optics and wireless systems, the comparable term is delay spread, which is due, respectively, to modal dispersion and multipath propagation. See also crosstalk, delay spread, lay length, modal dispersion, multipath propagation, skin effect, and twist ratio.