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In a fold profile, the point of highest elevation on a surface is called is called CREST or CRESTAL POINT and the one of lowest the TROUGH or TROUGH point. The hinge point and crestal point may coincide in a fold whose axial plane is vertical. A line joining the highest points on a single surface is called the CRESTAL LINE.

There could be an equivalent TROUGH LINE as well. On a folded surface in a fold profile, the point of least curvature is called the INFLEXION POINT. A line joining such points on a folded surface is called INFLEXION LINE. A plane containing crestal lines on a single folded surface is called an ENVELOPING SURFACE which may be planar or curviplanar. Similar plane containing trough lines is also called an enveloping surface. A plane containing all inflexion lines in a single folded surface is called the MEDIAN PLANE or surface.