In order to examine and discuss the use of the tonus peregrinus in the liturgies of the Western Church, some definitions of psalmody must be established. To do so, one of the regular psalm-tones and then an archetypal form of the tonus peregrinus will be used as examples. Only when a common terminology has been determined can discussion of the separate variants of the tonus peregrinus commence. This will be done primarily with the aid of selected characteristic examples drawn from liturgical and theoretical sources and secondly by reference to other sources with similar readings.