In musical notation, stems are the, "thin, vertical lines that are directly connected to the [note] head." Stems may point up or down. Different-facing stems indicate the voice for polyphonic music written on the same staff. Within one voice, the stems usually point down for notes on the middle line or higher, and up for those below. If the stem points up from a notehead, the stem originates from the right-hand side of the note, but if it points down, it originates from the left. There is an exception to this rule: if a chord contains a second, the stem runs between the two notes with the higher being placed on the right of the stem and the lower on the left. If the chord contains an odd numbered cluster of notes a second apart (such as C, D, E), the outer two will be on the correct side of the stem, while the middle note will be on the wrong side.