As an anisotropic particle between crossed linear polarizing filters is rotated in the plane of the stage by rotating the stage it generally goes dark every 90 degrees, but not always. If the particle goes dark (extincts) and has a characteristic morphology then its "extinction position" can be said to be parallel to a characteristic morphology (parallel extinction), bisect a characteristic angle (symmetric extinction), or, if neither of those conditions are satisfied, then it has oblique extinction. Some anisotropic materials don't change on rotation of the stage. Mature cotton fibers are an example. They have no extinction position. Other materials may darken but not go extinct. Peristerites are an example. Some materials shade into reddish, then bluish darker positions on rotation because the extinction position for red and blue wavelengths are not aligned. Wood fibers are a common example. Some particles consist of crystal structures that produce unique extinction patterns, such as starch and some minerals. Polymers often exhibit birefringence that results from stress, dander is an example.