The simplest physically realizable anisotropic system is hexagonal symmetry anisotropy, known as transverse isotropy (TI). Hexagonal symmetry has an axis of cylindrical symmetry, which implies that the properties of the medium in a given direction depend only on the angle between that direction and the symmetry axis direction. The symmetry direction is usually a result of some dominant global directed influence, such as gravity, regional stresses or parallel cracks. If gravity causes it, then the symmetry axis lies vertically, and the medium has transverse isotropy with a vertical axis of symmetry (VTI). If the direction of symmetry results from horizontal regional stresses or vertically parallel cracks, the symmetry axis lies horizontally, and the medium has transverse isotropy with a horizontal axis of symmetry (HTI).