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    The vast majority of optic nerves have a recognizable central depression or cup. An estimate of the ratio of cup diameter to disc diameter is best made vertically along an imaginary line drawn from the 12-o’clock to the 6-o’clock position through the center of the disc. The vertical disc diameter is arbitrarily set at 1.0 and then the relative cup diameter is estimated. The cup-to-disc ratio shown in this slide is 0.3. A cup-todisc ratio of 0.5 or less has a lower level of glaucoma suspicion.