What Makes a Prokeimenon "Great"?

This is currently being discussed on the choir director's list. Contrary to some people's opinions, a Great Prokeimenon (as opposed to a "prokeimenon" without the "great"), is not when the sopranos are all on the same note and the basses are not lagging a half-beat behind, though that would certainly make any choir director pretty happy.

The distinguishing characteristic of a Great Prokeimenon is multiple stichoi as opposed to a single stichos for a "regular" or "common" prokeimenon. Also, Great Prokeimena are found during Vespers only, and not during Divine Liturgy.

Although most people mean the Great Prokeimenon of Pentecost when they refer to the Great Prokeimenon (Who is so great a God as our God?), there are several other instances of Great Prokeimena during the liturgical year.

Great Prokeimena are found during the following Vespers:
Saturday Evening Vespers on Christmas Day, Dec. 25
on Theophany Day, Jan. 6
on Cheesefare Sunday
on the Sundays of the Great Fast (exclusive of Palm Sunday)
on Pascha & all the days of Bright Week
on Ascension Day
on Pentecost
on August 6, Transfiguration Day
on September 14, Exaltation of the Holy Cross

Although we classify Great Prokeimena as having multiple verses, there is also a hymnographic difference, particularly noticeable in Znamenny chant. The standard Prokeimena are sung to melodies that are generally formulaic, either according to the 8 Tones or according to a specific motif. However, the Great Prokeimena have more elaborate melodies which are more or less "unique" melodies for each text, and are written out individually in the traditional chant books. (One can almost say that Great Prokeimena are "Idiomela/Samoglasny" and standard Prokeimena are "Prosomoia/Podobny".)