Tantum Ergo

Tantum ergo is the incipit of the last two verses of Pange Lingua, a Medieval Latin hymn written by St Thomas Aquinas c. 1264. The Genitori Genitoque and Procedenti ab utroque portions are adapted from Adam of Saint Victor's sequence for Pentecost.
The singing of Tantum Ergo occurs during veneration and Benediction of the Blessed Sacrament in the Roman Catholic Church and other denominations that have this devotion. It is usually sung, though solemn recitation is sometimes done, and permitted.