Milan, the "second city" of medieval Italy after Rome, tenaciously clung to its local liturgical privileges. Legend credits the Milanese with defying even Charlemagne in defense of the so-called Ambrosian chant. This liturgy, specific to the diocese of Milan, spread to most of northern Italy and even some parts of present-day Austria; elements of the peculiar chant even survived the Council of Trent. Its name derives from St. Ambrose, the fourth century bishop of Milan and the supposed founder of the chant. At the very least, Ambrose was seminal for the early liturgical hymn, though hymns are strangely sparse in the Ambrosian repertory. The Ambrosian chant never grew to the bloated size of the pre-Tridentine repertory of Gregorian chant, but it maintained its own distinct musical traditions and liturgical forms.
Musically, many specific Ambrosian melodies in the Middle Ages also resisted the winds of change and may have traveled to later times in similar forms. Numerous other examples exist of Ambrosian chants that bear a filial relationship to Gregorian or Old Roman chants; often a process of musical embellishment clarifies that relationship. As with Gregorian chant, the Ambrosian melodies frequently display signs of motivic expansion by the addition of small melodic cells, perhaps indicating improvisation or oral transmission. The Ambrosian melodies tend to progress more often in stepwise motion than the Gregorian ones and span a much wider spectrum of style from one chant to another: extremely elaborate and solemn melismatic chants live alongside the simplest syllabic settings.
The forms and classifications of Ambrosian chants are sometimes parallel to those for Gregorian chant, but with prominent exceptions. The Ambrosian liturgical year follows only two main divisions, summer and winter (following an alternation between the two principal churches of medieval Milan). The Ambrosian Mass Ordinary only consists of a simply set Gloria, Credo, and Sanctus. Several of the distinctly Ambrosian Mass Propers (which also evolved from psalmodic antiphons) correspond to similar Gregorian forms: Ingressa (Introit), Post-Evangelium (Gradual), Cantus/Alleluia (Tract/Alleluia), Offrenda (Offertory), and Confractorium. Some elements of the Ambrosian Office expand upon the Gregorian liturgy: though the Ambrosian rite, for instance, conflates the Offices of Matins and Lauds, on festal days it adds an elaborate antiphona ad crucem during a mid-service procession, a Gloria patri, two processional psallendae, and a responsory in baptistero during concluding stational processions. Ambrosian Vespers, similarly, add an opening Lucernarium and on Sundays and feasts and an extra Vigil at the conclusion. ~ Timothy Dickey, Rovi