The numbers on your eyeglass prescription relate to the shape of your eyes and strength of your vision. They can help you figure out whether you have nearsightedness, farsightedness or astigmatism — and to what degree.

If you know what to look for, you can make sense of the numbers and abbreviations on your prescription chart.

Eye doctors use the abbreviations “OD” and “OS” to denote your right and left eyes. 

Your eye doctor may give you a paper prescription that looks something like this.

• OD is your right eye. OD is short for oculus dexter, the Latin phrase for “right eye.”
• OS is your left eye. OS is short for oculus sinister, Latin for “left eye.”

Your vision prescription may also have a column labeled "OU." This is the abbreviation for oculus uterque, which means "both eyes" in Latin.

These abbreviated terms are common on prescriptions for glasses,contact lenses and eye medicines, but some doctors and clinics have opted to modernize their eye prescriptions by using RE (right eye) and LE (left eye) instead of OD and OS.

The information for your right eye (OD) always comes before the information for your left eye (OS). Eye doctors write prescriptions this way because, when they face you, they see your right eye on their left (first) and your left eye on their right (second).

Sphere indicates the amount of lens power prescribed to correct nearsightedness or farsightedness. Lens power is measured in diopters (D).

• If the number under this heading comes with a minus sign (–), you are nearsighted.
• If the number under this heading has a plus sign (+), you are farsighted.

The term "sphere" means that the correction for nearsightedness or farsightedness is "spherical," or equal in all meridians of the eye.

Cylinder indicates the amount of lens power needed for astigmatism. It always follows the sphere power on an eyeglass prescription.

The number in the cylinder column may have a minus sign (for correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism).

If nothing appears in this column, you either don’t have astigmatism, or your degree of astigmatism is so small that it doesn’t need to be corrected.

The term "cylinder" means that this lens power added to correct astigmatism is not spherical, but instead is shaped so 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.

Meridians of the eye are determined by superimposing a protractor scale on the eye's front surface. The 90-degree meridian is the vertical meridian of the eye, and the 180-degree meridian is the horizontal meridian.

Axis describes the lens meridian that contains no cylinder power to correct astigmatism.

If an eyeglass prescription includes cylinder power, it also needs to include an axis value, which follows the cylinder power.

Meridians of the eye are determined by superimposing a protractor scale on the eye's front surface. The 90-degree meridian is the vertical meridian of the eye, and the 180-degree meridian is the horizontal meridian.

The axis is defined with a number from 1 to 180.

• The number 90 corresponds to the vertical meridian of the eye.
• The number 180 corresponds to the horizontal meridian of the eye.

The axis is the lens meridian that is 90 degrees away from the meridian that contains the cylinder power for astigmatism correction.

“Add” is the added magnifying power applied to the bottom part of multifocal lenses to correct presbyopia — the natural farsightedness that happens with age.

The number appearing in this section of the prescription is always a "plus" power, even when you don’t see a plus sign. Generally, it will range from +0.75 to +3.00 D and will be the same power for both eyes.

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

Only a small percentage of eyeglass prescriptions include a prism measurement.

When present, the amount of prism is indicated in either metric or fractional English units (0.5 or ½, for example), and the direction of the prism is indicated by noting the relative position of its "base" (thickest edge).

Four abbreviations are used for prism direction: BU = base up; BD = base down; BI = base in (toward the wearer's nose); BO = base out (toward the wearer's ear).

Sphere power, cylinder power and add power always appear in diopters. They are in decimal form and generally are written in quarter-diopter (0.25 D) increments.

Axis values are whole numbers from 1 to 180 and signify only a meridional location, not a power.

When prism diopters are indicated in decimal form, typically only one digit appears after the period (e.g., 0.5).

An example eye prescription chart

Still confused? Let's look at an example prescription chart:

In the right eye (OD), the eye doctor prescribed:

• -2.00 D sphere for the correction of nearsightedness.
• No cylinder power or axis, which means no astigmatism is present. This doctor chose to write "SPH," to confirm the right eye is being prescribed only spherical power. Some doctors will add "DS" for "diopters sphere" and others will leave it blank.

The left eye (OS) was prescribed:

• -1.00 D sphere for nearsightedness correction.
• -0.50 D cylinder for the correction of astigmatism.
• A cylinder power with an axis at the 180 meridian. This means that the horizontal (180-degree) meridian of the eye has no added power for astigmatism and the vertical (90-degree) meridian gets the added -0.50 D from the cylinder column.

Both eyes were prescribed:

• An "add power" of +2.00 D for the correction of presbyopia.
• Prismatic correction of 0.5 prism diopter in each eye. In the right eye, the prism is base down (BD); in the left eye, it's base up (BU).

Your eye doctor may also write specific lens recommendations on your eyeglass prescription. They might suggest anti-reflective coating, photochromic lenses and/or progressive lenses to give you the most comfortable vision correction possible.

SEE RELATED: How “bad” is my prescription?

No, you cannot use your glasses prescription to buy contact lenses.

An eyeglass prescription only works for the purchase of eyeglasses. It does not contain certain information that is crucial to acontact lens prescription.

That information can only be obtained through a contact lens fitting, an additional procedure that can be performed during your eye doctor visit.

Eyeglass lenses are positioned at a distance from the eyes, while contacts rest directly on the eyes. That distance affects the lens power required for eyes to focus properly.

In addition to the information in an eyeglass prescription, a contact lens prescription must specify the base (central) curve of the back surface of the contact lens, the lens diameter, and the specific manufacturer and brand name of the lens.

Also, the power of an eyeglass prescription frequently is modified when determining the best contact lens power. This is because eyeglass lenses are worn some distance (usually about 12 millimeters) from the surface of the eye, whereas contact lenses rest directly on thecornea of the eye.

An accurate contact lens prescription can be written only after a contact lens fitting has been performed and the prescribing doctor has evaluated your eyes' response to the lenses and to contact lens wear in general.

SEE ALSO:Contact lens vs. eyeglasses prescriptions

The Federal Trade Commission (FTC) is the U.S. government's consumer protection agency. Their Prescription Release Rule requires that eye doctors give patients a copy of their eyeglass prescription at the end of an eye exam that includes a refraction.

The Prescription Release Rule is intended to allow the "portability" of your eyeglass prescription, giving you the freedom to buy glasses from any vendor of your choice.

Your eye doctor must give you a copy of the prescription whether or not you ask for it. Eye doctors may not condition the release of your prescription on your agreement to purchase eyeglasses from them, nor may they charge you an extra fee to release your prescription.

If you think your eye doctor has violated this rule, you can report the problem to the FTC.