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A prescription is composed of three (3) numbers:
The first number (for example, -3.25 diopters) indicates the degree of either nearsightedness or farsightedness. A minus sign (- sign) means that you are nearsighted (have myopia) and a plus sign (+ sign) means that you are farsighted (have hyperopia).
The second number (for example, -2.00 diopters) indicates the degree of astigmatism. Optometrists generally write this second number with a minus sign (- sign).
The third number (for example, x 90) identifies the axis, which indicates the direction of your astigmatism. The axis can be between 1 and 180, and is like the points of a compass. An axis of 180 degrees means the astigmatism is horizontal, while an axis of 90 degrees means the astigmatism is vertical and an axis of 45 or 135 degrees means the astigmatism is oblique.
Therefore, a prescription of [-3.25-2.00x90] indicates that the patient is moderately nearsighted, and has a significant degree of astigmatism that is oriented in a vertical direction.
If you want to understand more about the terms myopia, hyperopia, astigmatism, and diopters, click here.