Trisagion

The Trisagion (Greek: Τρισάγιον "Thrice Holy"), sometimes called by its opening line Agios O Theos or by the Latin Tersanctus, is a standard hymn of the Divine Liturgy in most of the Eastern Orthodox Churches, Oriental Orthodox Churches and Eastern Catholic Churches. In Western Christianity the Sanctus (a different formula) is used in the Mass.

In those Churches which use the Byzantine Rite, the Trisagion is chanted immediately before the Prokeimenon and the Epistle Reading. It is also included in a set of prayers named for it, called the Trisagion Prayers, which forms part of numerous services (the Hours, Vespers, Matins, and as part of the opening prayers for most services).