Euouae

Euouae or Evovae is a mnemonic which was used in medieval music to denote the sequence of tones in the `seculorum Amen` passage of the lesser doxology, Gloria Patri, which ends with the phrase In saecula saeculorum, Amen. In plainchant sources, the differentia, that is, the melodic formula to be sung at the end of every line of chanted psalmody,.....
Found on http://en.wikipedia.org/wiki/Euouae

EUOUAE

the vowels from the words 'seculorum amen,' the final words of the doxology. The abbreviation is typically supplied with musical cues to show the singer how to link the end of the psalm tone and/or doxology (which use the same recitational melody) to the beginning of the antiphon in the introit.
Found on http://people.vanderbilt.edu/~cynthia.cyrus/ORB/orbgloss.htm
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