Did you ever wonder what all those numbers and abbreviated terms on your eyeglass prescription meant? If you’d like to be able to decipher all the parts of your prescription so you can discuss it intelligently with your optician, read on.

What Do OD and OS Mean?

OD stands for oculus dexter and OS stands for oculus sinister, Latin terms that simple mean right eye and left eye, respectively. The Latin term for both eyes is oculus uterque, represented on your prescription as OU.

These are the traditional terms used for writing prescriptions for eyeglasses, contact lenses and eye medicines, however, some doctors and clinics have opted to modernize by using RE for right eye and LE for left eye.

Information for your right eye comes before the information for your left eye on the prescription because your optometrist sees your right eye first and your left eye second when they face you during an eye exam.

Other Terms on Your Eyeglass Prescription

Other terms and abbreviations on your eyeglass prescription include the following:

Sphere (SPH), indicates the amount of lens power, measured in diopters (D), prescribed to correct nearsightedness or farsightedness. A minus sign under this heading means that you are nearsighted and a plus sign under it or not preceded by a plus or minus sign, means you are farsighted.

The term sphere means that the correction for nearsightedness or farsightedness is equal in all meridians of the eye. Meridians of the eye are determined by superimposing a protractor scale on the front surface of the eye.

Cylinder (CYL) indicates the amount of lens power for astigmatism. If you don’t see this term on your prescription, either you don’t have astigmatism or it is so slight that it’s not necessary to correct it with your eyeglasses. Cylinder power always follows sphere power in an eyeglass prescription.

The term cylinder means that the lens power added to correct astigmatism is not spherical but is shaped so that one meridian has no added curvature and the meridian perpendicular to this “no added power” meridian contains the maximum power and lens curvature to correct astigmatism.

The number in the cylinder column may be preceded with a minus sign for the correction of nearsighted astigmatism or a plus sign for farsighted astigmatism.

Axis describes the lens meridian that contains no cylinder power to correct astigmatism and is defined with a number from 1 to 180. The number 90 relates to the vertical meridian of the eye and the number 180 relates to the horizontal meridian.

If your eyeglass prescription includes cylinder power, it must also include an axis value. The CYL power is preceded by an “X” when written freehand.

Add is the added magnifying power applied to the bottom part of multi-focal lenses to correct presbyopia. This number is always a “plus” even when not prefaced by a plus sign. It usually ranges from +0.75 to +3.00 D and will be the same power for both eyes.

Prism is the amount of prismatic power prescribed to compensate for eye alignment problems. It is measure in prism diopters, (p.d., or a superscript triangle when written freehand.)

Sphere power, cylinder power and add power always appear in diopters in decimal form and are usually written in 0.25D increments.