When Pope Urban IV extended the feast of Corpus Christi to the whole church in 1264, tradition records that the office and Mass texts for the new feast were entrusted to Thomas Aquinas, and the liturgy that resulted is commonly counted the most perfect ever composed for a single feast. The Pange Lingua is its Vespers hymn. In six stanzas Aquinas narrates and interprets the eucharist, the word made flesh who at the last supper gave himself to the twelve, fulfilling the law with his own hands, and then the couplet on which the whole theology balances, the word as flesh makes true bread flesh by a word, and if sense fails, faith alone suffices to steady the sincere heart.
The hymn ends by falling on its knees. Its last two stanzas, beginning Tantum ergo sacramentum, let us venerate so great a sacrament with bowed heads, became a liturgical unit of their own, sung at Benediction of the Blessed Sacrament wherever that devotion exists, so that millions who have never sung the full hymn know its ending by heart. On Holy Thursday the full Pange Lingua accompanies the procession that carries the sacrament to the altar of repose, with the Tantum Ergo reserved for the arrival, the one night of the year when the hymn does exactly the work it was written to do.