Fortunatus is best known for two poems that have become part of the liturgy of the Catholic Church, the "Pange lingua gloriosi proelium certaminis" ("Sing, O tongue, of the glorious struggle"), a hymn that later inspired St Thomas Aquinas's "Pange Lingua Gloriosi Corporis Mysterium". He also wrote "Vexilla Regis prodeunt" ("The royal banners forward go"), which is a sequence sung at Vespers during Holy Week. This poem was written in honour of a large piece of the True Cross, which explains its association also with the feast of the Exaltation of the Holy Cross. The relic had been sent from the Byzantine Emperor Justin II to Queen Radegund of the Franks, who after the death of her husband Chlotar I had founded a monastery in Poitiers. The Municipal Library in Poitiers houses an 11th-century manuscript on the life of Radegunde, copied from a 6th-century account by Fortunatus.