Faults are discontinuities in rocks; they are surfaces along which movement has taken place. An estimation of the amount of movement (net slip) that has occurred is sometimes possible on ancient faults by the matching of once coincident features across the fault. The features that are required for this purpose need to define points on the fault surface, one on each wall of the fault plane, which from their off-set allows the net slip to be measured (points p, p′ in Fig. 28a). In rare circumstances these points could be small objects (e.g. clasts in a conglomerate) displaced by the fault. More often the necessary points are defined by the mutual intersection point of three planes: the fault itself plus two other non-parallel markers.

In Figure 28a, two non-parallel planar markers (beds, sheet intrusions, unconformities) meet along i, their line of intersection. (Movement on the fault has meant that line i is not continuous but now consists of two parts i and i′.) Points p and p′ are located where line i is cut by the fault and allow the measurement of net slip (Fig. 28a). In some circumstances it might be convenient to consider the dip-slip and strike-slip components of the net slip.

The data required for the calculation of net slip are shown in Fig. 28b, which is a map of the fault and of two displaced planar markers (a, b).