CYL or cylinder power indicates the severity of astigmatism. The cylinder is part of vision correction but is not necessary for everyone. If the eye is usually round, then in the case of a cylinder it is more melon-shaped. Seeing images and text double or with a shadow is a sign of a cylinder. Together with cylinder power, an axis (ax) indicating the positioning of the cylinder correction is also determined. On an eyeglass prescription, the cylinder is referred to as astigmatism, whereas in the case of contact lenses, they are called toric contact lenses.