1.    Plainsong
        A monophonic style of sacred song used in Western Christian worship; characterized by non metrical rhythms and modal scale structure.

2.    Trope
        The addition or insertion of a text or music, or both, into a preexistent composition, especially plainsong. 

4.    Florid Organum  [Organum Duplum/Organum Triplum]
        A melismatic type of organum where the original plainchant melody (played or sung) lies always in the lower voice, but each note is prolonged so as to allow the upper voice to sing phrases of varying length against it.

5.    Clausula
        A metrical "clause" within a piece of florid organum. 

7.    Polyphonic Conductus
        A 13th century composition in which the cantus firmus (fixed voice)  is original and not borrowed from plainsong.  It is also identified by the fact that all parts move in the same basic rhythm.  (This style represents true choral polyphony;  one of the first wholly original polyphonic styles in Western music.

8.    Organum Quadruplum  [and Organum Triplum]
        An important innovation made by Perotin and his contemporaries was the expansion of organum from two voices to three and four voices.  Since the second voice had been called "duplum", by analogy the third and fourth were called respectively the "triplum" and "quadruplum". 

9.    Early Motet
        Eventualy, the clausula 'cut loose' from the larger organum in which it had been embedded and began life on its own as a separate composition.  Probably because of the addition of words, the newly autonomous pice was called a motet (French = 'mot').

10.  Franconian Motet
        A 13th century form in which each voice is given a different rhythmic mode.

11.  Petronian Motet
        A 13th century form in which the upper voice is given more freedom and often moves with the rhythm of the words.

12.   Isorhythmic Motet
        A 14th century form which utilizes rhythmic and/or melodic patterns (repeated) in its tenor and/or all other parts.