Page "Intersymbol interference" Paragraph 7

One way to study ISI in a PCM or data transmission system experimentally is to apply the received wave to the vertical deflection plates of an oscilloscope and to apply a sawtooth wave at the transmitted symbol rate R ( R = 1 / T ) to the horizontal deflection plates.

The resulting display is called an eye pattern because of its resemblance to the human eye for binary waves.

The interior region of the eye pattern is called the eye opening.

An eye pattern provides a great deal of information about the performance of the pertinent system.