The distinction between separation and slip is fundamental to the proper geometric understanding of fault displacements. Ideally geologists strive to find slip: the relative displacement of formerly adjacent points on opposite sides of a fault. In practice we recognize these points where lines formed by geological elements meet the fault plane at piercing points. Such lines are primarily those formed: (1) by intersecting planes, such as dikes transecting strata; (2) by the trace of one plane against another, as where a bed meets an unconformity; (3) by linear geological features, such as attenuated sand lenses, ore shoots, and stream courses; (4) by stratigraphic lines, such as pinch-out lines, lines of facies changes, and fossil shorelines; (5) by constructed lines, such as isopachs, lithofacies lines, and traces of axial surfaces with bedding.

Not always are such data in three-dimensional space available to determine slip. Usually we have only information on displaced planes, such as bedding planes, unconformities, dikes, sills, contacts, etc., and lack recognizable lines lying within these planes. Furthermore, data are commonly given in two dimensions only. More widespread recognition that we must describe the geometry of fault displacements in terms of separation is therefore necessary. Moreover, since separation measures the displacement of traces of displaced planes as shown on a cross section or map, it is as essential to define the orientation of this view as to describe and give the location, amount, and sense of the separation. Where we employ the term apparent in fault definitions, in general we refer to separation

Geologists must therefore distinguish habitually between geological situations with displaced lines and those with displaced planes. Our fault practice and terminology fail to draw this distinction sharply, so that faults are inadvertently described incorrectly when slip terms are applied carelessly to separation. A qualified committee of geologists should now examine our fault nomenclature and make recommendations that will dispel ambiguities. Precise definitions would also stimulate the search of geological data for clues to slip that alone can reveal significant facts on deformation kinematics.