Euouae 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, would be written over either the letters EUOUAE, or merely E----E, representing the first and last vowel of "seculorum Amen.