In structural geology, vergence refers to the direction of the overturned component of an asymmetric fold. In simpler terms, vergence can be described as the horizontal direction in which the upper-component of rotation is directed. Vergence can be observed and recorded in folds to help a geologist determine characteristics of larger fold areas. Vergence is used to provide an overall characterization, in the symmetry (or asymmetry) of folds, and can be used to observe changes in small-scale structures in relation to the axis of a large fold. The vergence of a fold lies parallel to the surrounding surfaces of a fold, so if these surrounding surfaces are not horizontal in nature, the vergence of the fold will be inclined. For a fold, the direction, as well as the extent to which vergence occurs, can be calculated from the strike and dip of the axial surfaces, along with that of the enveloping surfaces. These calculations can be very useful for geologists in determining the overall elements of larger areas. Historical Confusion on the Definition of Vergence Vergence is a term that has been commonly confused and, as a result, somewhat misunderstood throughout its history of use. The earliest form of use, and the recognized introduction to the term vergence, came from the German geologist, Hans Stille, in 1924. Stille originally used the term to describe the direction of the overturning of minor folds, as well as to describe the "up-dip" direction, which is directly related to the strike and dip of a fold. These descriptions of certain physical components of a fold, determined by vergence, were used by Stille in the description of the overall direction of the over-thrusting in a fold and, as a result, they are directly related to the kinematic components in a fold. In this sense, the term vergence, has implications of movement and, thus, has become controversial in geology, because vergence, in Stille's original definition, only describes geometrical relationships, and many geologists believed it should not be used as a primary tool for describing movement. As a result of this controversy, the use of the term never reached international agreement, and consequently has been used in distinctly different ways, throughout history. Most of the confusion, about the definition of vergence, stems from the confusion and conflation of the concepts of fold-facing and fold-vergence. Some geologists began to reference the definition of vergence to being the direction