This piece, a short antiphon from the Midday Office, gives us a clear picture of Mode 3. The Reciting Tone – TI, is circled in green. It is the prominent pitch at the beginning of the antiphon. The first phrase gently descends to below the Home Tone, MI, circled in red. The final phrase is a “classic” Mode 3 ending. The characteristic half-step – FA-MI has been marked with a caret (^).
Listen for these key pitches. The other thing to note is that for a relatively short piece, it covers almost an octave range. Mode 3 pieces are usually more expansive. In the section following the end of the antiphon, you’ll see some pitches that revolve around the Reciting Tone, and some vowels (Euouae) beneath the notes. This is the Psalm Tone Ending that goes with this particular antiphon, and the vowels are a short cut for the last half of the Gloria Patri verse, sung at the end of every Psalm. (Saeculorum, Amen.)