All prescription lenses have what's called an Optical Center (OC).  This is the most accurate part of the lens and in most cases it is placed right in front of the wearers pupils.  For this reason, a Pupillary Distance measurement is important on all prescription glasses.

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All lenses have a front and back surface but in the case of prescription lenses those surfaces do not run parallel to each other.  

On a minus prescriptions (Nearsighted) the OC will be found in the thinnest part of the lens.  With a plus prescription (Farsighted) the OC is found at the thickest point.

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Another way to visualize this is by thinking about two triangles; if you placed them base to base the OC would be located at the point where they meet and same would go if they were placed point to point.