The Trisagion, the thrice-holy, is the hymn "Holy God, Holy Mighty, Holy Immortal, have mercy on us," sung near the beginning of the Byzantine Divine Liturgy and used widely in Eastern Christian worship. Threefold in its address, it is distinct from the Sanctus of the eucharistic prayer, though both take up the theme of holiness, and the Eastern tradition surrounds it with an account of its origin in a revelation given during an earthquake at Constantinople, when a child was said to have been caught up and taught the angelic hymn.
A single disputed addition to it, the phrase "who was crucified for us", became a point of sharp controversy in the fifth and sixth centuries, bound up with debates over how the divine and human were united in Christ. Sung repeatedly and with reverence, the Trisagion is one of the fixed and beloved hymns of the Orthodox liturgy and is also used in the Western church, notably in the Good Friday reproaches.