A line drawn parallel to horizontal lines tangent to the lowest and highest edges of a lens is called what?

The term that refers to a line drawn parallel to horizontal lines tangent to the lowest and highest edges of a lens is known as the datum line. The datum line serves as a reference point in the process of fitting and aligning lenses, ensuring consistency in measurements and the overall design layout. It is particularly important in ophthalmic dispensing as it helps in positioning the optical center of the lens at the appropriate height for the wearer, thereby improving visual performance and comfort.

This concept is integral in providing a stable reference for further measurements and adjustments, such as alignment with the frames or ensuring the lenses sit correctly in relation to the wearer's pupils. In contrast, the other choices do not accurately capture this specific function in lens design and fitting, which is why they are not relevant to the description of the line in question.