The various types of planar structure that occur in rocks are listed on p. 2. Cleavage is an example of a pervasive planar structure induced by tectonic strains (changes of shape) coupled with metamorphic modifications of a rock's fabric. An alignment of grains visible microscopically is often expressed on the scale of the hand specimen by the rock's ability to break along parallel planes. The direction perpendicular to these cleavage planes is the direction of greatest shortening strain. It is frequently observed that the shortening strains giving rise to cleavage also result in the folding of other planar structures such as bedding. Folding and cleavage are therefore often associated.

Cleavage is frequently parallel or sub-parallel to the axial surfaces of associated folds and in such cases is referred to as axial-plane cleavage (Fig. 26a). As a consequence the lines of intersection of folded bedding planes with the cleavage plane are parallel to the fold axis (Fig. 26b). Field measurements of cleavage provide therefore not only an indication of the orientation of fold axial planes but also, in combination with bedding, information on the plunge and plunge direction of fold axes. Bedding–cleavage intersection lineations can be measured directly in the field or constructed stereographically from the measured attitudes of bedding and cleavage (see p. 26).

Axial-plane cleavages are not often exactly parallel to fold axial surfaces. The strains around the fold are variable in amount and direction because of competence contrasts between the folded beds.