Eye diagram

The idea is to trigger the oscilloscope on the clock and let every transition overlap, after which imperfections become geometry. The eye's height is the noise margin, its width is the timing margin, and the ways the edge smears apart differ characteristically: intersymbol interference produces distinct curves depending on the preceding bit, while random jitter produces an even smear that grows in the tails. Acceptance is judged against a mask, a forbidden polygon in the middle that no sample may cross, and for low error rates a bathtub curve is extrapolated instead, since you cannot measure for long enough. On modern links the measurement has moved into the silicon: the signal on the board is often entirely closed before the receiver's equalisation has done its work, so the only meaningful eye is the one that can be read out internally.