Cyclopedia of Biblical, Theological and Ecclesiastical Literature
(Ambrosianus titus, or Mediolanensis Ecclesice ritus), the office used in the Church of Milan (called the Ambrosian Church). Before the time of Charlemagne every Church had its own particular. office, and there is good reason to believe that this office was in use in the Church of Milan before the time of St. Ambrose; but that when the popes compelled all the Churches of the West to adopt the Roman office, the Milanese gave the name of St. Ambrose to theirs, in order to protect it, although he was not really the author of it. It may be, however, that St. Ambrose made alterations in the original office, which he found established in his Church; and the name of the office of St. Ambrose has been retained, to distinguish it from the Roman office in use in other churches. See Le Brun, Exp. de la Messe, 2, 176. (See AMBROSIAN LITURGY).
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McClintock, John. Strong, James. Entry for 'Ambrosian Office'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. https://www.studylight.org/encyclopedias/eng/tce/a/ambrosian-office.html. Harper & Brothers. New York. 1870.