Differentia
The closing formula of the psalm tone.
What it means
The differentia is the closing formula of a psalm tone. In liturgical psalm singing, the text is delivered on a reciting note framed by fixed turns: a beginning, a mediant cadence, and precisely the differentia at the end of the verse. For each psalm tone there are several such closing formulas that differ in the last notes. Which of them is chosen depends on the antiphon that frames the psalm: the formula leads melodically to its opening note.
In the notation
In liturgical books, the differentiae stand as short note sequences after the indication of the psalm tone, designated with letters or numerals. At the antiphon it is noted which formula is to be used. The distribution of syllables is indicated by emphases in the text so that the stressed syllables fall on the right notes.
Musical use
This system solves a practical problem: psalms are of different lengths and are sung in many different contexts. Instead of composing every verse separately, a basic model is used that adapts to any text. The differentia ensures the smooth connection to the return of the antiphon, so that the alternation between psalm verse and frame remains musically coherent. The procedure is an early example of how formulas and models are used flexibly in music.
Performance practice
Before singing, determine the psalm tone and the corresponding differentia and rehearse the transition to the antiphon, because the most common mistakes arise there. Distribute the syllables according to the markings in the book and not by feeling; what matters is that the stressed syllables lie on the stressed notes. Keep the reciting note calm and even and sing the formulas without delay so that the flow is preserved. In alternating singing, agree which group takes which verse.
Bear in mind that the choice of closing formula is not left to taste: it follows the antiphon, and a wrong formula leads to an unsuitable connection. Check the indication in the liturgical book and enter it. Practise the transition to the antiphon separately. Watch for shared distribution of syllables, because deviations in the group become audible at once.