This information is typically written in a grid with the abbreviations SPH, CYL, AXIS, and PRISM written across the top row, and with ADD, OD, OS, and OU written down the left column. The numbers listed under the headings of OS and OD are Latin abbreviations: OS (oculus sinister) means the left eye and OD (oculus dextrus) means the right eye. Occasionally, you will see a notation for OU, which means something involving both eyes. ADD is added magnifying power in the bottom of a multi-focal lens to correct presbyopia, the inability to focus on close objects.
PH (or Sphere) indicates the prescription power, or how strong your lenses need to be to correct your vision. For an indicator of how much magnifying power is needed in a bifocal or progressive lens to correct your vision problems, look for ADD.
CYL (or Cylinder) denotes astigmatism, a condition where the cornea is irregularly shaped causing blurred or distorted vision. It also tells the lens strength needed to fix it.
AXIS describes the degree and direction of your astigmatism.
PRISM tells you the amount of prismatic power needed to correct eye alignment problems. However, only a small percentage of eyeglass prescriptions have this.
In general, the further away from zero the number on your prescription, the worse your eyesight and the more vision correction you need. A plus sign in front of the number means you are farsighted and a minus sign means you are nearsighted. These numbers represent diopters, the unit used to measure the correction, or focusing power, of the lens your eye requires. Diopter is often abbreviated D. During your annual comprehensive eye exam your eye doctor will be able to tell you if you are due for a prescription update.
Eyeglass and contact lens prescriptions aren't the same. An eyeglass prescription is for the purchase of eyeglasses only. It does not contain certain information that is crucial to a contact lens prescription and that can be obtained only during a contact lens consultation and fitting.
This article originally appeared on Think About Your Eyes