C. In vocal music, to show that two or more tones are to be sung to one syllable of text. See Fig. 35.
In notes of small denomination (eighths and smaller) this same thing is often indicated by stroking the stems together as in Fig. 36. This can only be done in cases where the natural grouping of notes in the measure will not be destroyed.
D. To mark special note-groups (triplets, etc.), in which case the slur is accompanied by a figure indicating the number of notes in the group. See Fig. 37 (a)
The most common of these irregular note-groups is the triplet, which consists of three notes to be performed in the time ordinarily given to two of the same value. Sometimes the triplet consists of only two notes as in Fig. 37 (b). In such a case the first two of the three notes composing the triplet are considered to be tied.
