When you check out your nearsighted prescription, you will notice numbers listed under headings OS and OD. These are Latin abbreviations. OS or oculus sinister means your left eye and OD or oculus dextrus means the right eye. On occasion, you will see an OU notification, meaning something that involves both the eyes.

A minus sign means you are nearsighted and these numbers represent diopters. If your prescription reads -1.00, this means you have a diopter of nearsightedness. If it is -4.25, this means you have four and ¼ diopters and requires thicker or stronger lenses. A lot of people think that a +1.75 is required when buying over-the-counter reading glasses. However, it takes a bit of algebra to figure out the total power needed for single vision reading eyeglasses.

More abbreviations you have to know in order to understand your nearsighted prescription better includes SVD, SVN, and Sphere, Cylinder, PD, axis and prism. SVD means single vision distance or glasses that are for distance use only; SVN is single vision near or for reading only. Sphere is spherical power that has the same power in all meridians, cylinder power that corrects astigmatism and represents the difference in the greatest eye power and the weakest eye power, normally separated by 90 degrees. Axis indicates an angle between the two meridians of astigmatism, PD is the measurement vital in designing glasses that are comfortable to wear and optically good. Prism is often prescribed to displace an image in a certain direction for those with cross-eyes or other eye muscle disorders.