3. When two parts are written on the same staff, the stems of the upper part all turn up, and those of the lower part turn down, in order that the parts may be clearly distinguished. (Fig. 2.) But in music for piano and other instruments on which complete chords can be sounded by one performer and also in simple, four-part vocal music in which all voices have approximately the same rhythm, several notes often have one stem in common as in Fig. 3.
4. Notes of small denomination (eighths and smaller) are often written in groups of two or more, all stems in the group being then connected by one cross-stroke. In such a case all the stems must of course be turned the same way, the direction being determined by the position of the majority of note-heads in the group. Notes thus stroked may be of the same or of different denomination. See Fig. 4.
In vocal music notes are never thus stroked when a syllable is given to each note. (See [p. 19], [Sec. 55], C.)