Sedimentary rocks were originally deposited as horizontal beds, but deformation processes have translated, rotated and distorted most of these beds over the course of tectonic history. The current orientation of these planar features, such as tilted layers, can be measured and expressed in terms of azimuth, strike and dip. Terms introduced in the first paragraph include true dip, plunge, strike and azimuth. Secondly true dip is distinguished from apparent dip and finally true and apparent thickness are closing off this chapter.