Here's a little paper I wrote on the subject. If you choose to use this as a reference, please cite it correctly. Also, please leave a comment to acknowledge your use of my article! Thanks!
The Origin of the Sequence and the Trope
by Ruth Seiler
In the year 862, a young monk arrived at the monastery of St Gall in Switzerland, having left his native Normandy due to troubles there. This young man brought with him a volume of chants to which had been added texts to the long melismas that make up the Jubilus of the alleluias. Notker Balbulus, upon seeing this technique, recognized it as a useful way to help singers remember the long jubilus melodies and used his skill as a poet to fit Latin texts to the melodies. According to Albert Seay, Notker worked “from the principle that the original melismata should be broken down into individual notes and each not thus derived provided with a syllable.” (Seay, pg 50)
The sequence evolved from adding text to a pre-existing melisma to an independent, originally poetic, form. The structure of the sequence remained fairly consistent through its history: After a single introductory line, each melodic line was set syllabically with a versicle and repeated with a different but related versicle, then new melodic material would be introduced with another pair of versicles. This pattern of paired melodic lines would end with a single versicle set to new melodic material again.
Form of Sequence:
X – AA – BB – CC ……. - Y
Sequences were usually sung after the Alleluia in the Mass and kept its “melodic couplet idea, with new musical material for each pair of verses.” (Seay, pg 51) They became very popular and individualistic from one church to another, which caused the Council of Trent to remove most sequences from the Liturgy. The ones that remain are “Victimae paschali laudes,” “Veni sancta spiritus,” “Lauda Sion,” “Dies irae,” and “Sabat Mater,” which was reintroduced in the eighteenth century.
A contemporary and friend of Notker Balbulus at St. Gall – Tuotilo, introduced the trope. A trope was an addition to a preexisting chant. There were three different types of troping: 1) The addition of melodic material. This means adding melismas at the end of a chant phrase. 2) The addition of text to a preexisting melody. For example, text could be added to melismas, or text could be added to a previously added melodic trope. 3) Both new text AND melodic material could be added at once. According to the Harvard Dictionary of Music, the trope texts “served the purpose of amplifying and interpreting the receive texts of the liturgy and making the ancient words relevant to the needs and understand of the contemporary listener – that is, the monastic communities…” (Harvard, pg 876) It is not clear how tropes were performed, but they flourished from the 10th to the 12th centuries. Tropes were not purged from the liturgy during the Council of Trent in the same manner as sequences.
Seay, Albert. Music in the Medieval World. Prentice-Hall, Inc. Englewood Cliffs, NJ: 1965
Randel, Don, editor. The Harvard Dictionary of Music. The Belknap Press of Harvard University Press. Cambridge, Massachusetts : 1986