The prose text added to the respond connects the chant directly to the feast day the Mass is celebrating , the birth of Jesus , by telling the Christmas story in miniature . In addition , several of the accented or prominent vowels in the prosula echo the vowels in the original text set to the same notes , as shown by underlining here and on page 36 : the " ah " from " Al - " ( sung to the first two notes of the chant ) is present in “ Na - " : the " eh " from " - le - " ( sung to the next seven notes ) is in " est " and " hodie ” : the " 00 " from " - lu - " ( sung to the next seven notes ) is in “ parvulus filius ” ; and the " ah " from the closing syllable " - a " appears ten times in the rest of the text on accented syllables or important words . This resemblance in sound between the source text and the new text , known as assonance , is common in tropes and later in motets ( see NAWM 20 ) .