Pange Lingua is a hymn written by St. Thomas Aquinas (1225-1274) for the Feast of Corpus Christi (now calle the Solemnity of the Holy Body and Blood of Christ). It is also sung on Holy Thursday, during the procession from the church to the place where the Blessed Sacrament is kept until Good Friday. The last two stanzas, called separately Tantum Ergo, are sung at Benediction of the Blessed Sacrament. The hymn expresses the doctrine of transubstantiation, in which, according to the Catholic faith, the bread and wine are changed into the Body and Blood of Christ.