Your glasses prescription has several different components. In most cases, you will see OD and OS written on your prescription. OD stands for your right eye, while OS stands for your left eye. The first column on your prescription indicates how much spherical power you need to see clearly. Typically, this refers to your refractive error. This could be a plus prescription or a minus prescription.
There may also be a cylinder and axis component to your glasses prescription. It is prescribed to patients who have astigmatism, which is an irregular front surface of the eye. To correct your astigmatism, you need two different powers.
The ADD component is also part of the glasses prescription for patients who need some sort of reading component in addition to their glasses prescription. ADD power is typically plus power, which allows you to see clearly up close. Patients are typically prescribed this when they approach presbyopia. Presbyopia refers to the condition of losing the ability to focus after 40 years of age. Additionally, children can also have an ADD component to their glasses. If they have difficulties focusing up close, their eye doctor may write an ADD power to their glasses, or if they have a particular eye deviation, their eye doctor may think it's necessary to include an ADD power as well. In the case of myopia, progressive glasses may also be prescribed that include ADD power to help slow the progression of their myopia.
Your glasses prescription may also contain a prism and a base component. It is prescribed for patients who may have an eye deviation and exhibit symptoms of double vision. A prism can help reduce symptoms of double vision. The prism indicates how many units you need of that prism diopter, and the base indicates which direction the lab will incorporate the prism into your glasses to get rid of any symptoms of double vision. So that's what you would typically see in your glasses prescription.
Contact lens prescription differs from prescription for glasses. Your contact lens prescription might include different powers for astigmatism and spherical powers, as well as the base curve that describes the curvature of the lenses, since every pair of lenses has a different base curve. Diameter is another component, which indicates how large contact lenses are.
The prescription also specifies the brand of the contact lens, what type the patient should wear, whether it should be changed daily, every two weeks, or once a month. There is a lot that goes into your contact lens prescription, which is why it's very important to have a proper contact lens evaluation and proper fitting since each contact lens is different.
Specialty lenses have even more components to the prescription. They may have a peripheral curve radius or optic zone diameters, which is somewhat more complex and requires specialized care because it is a specialty design lens.