Septuplet
A septuplet is a figure of 7 notes in the time of usually 3 or 4 notes.
7 achtereenvolgende septuplet notes together form a septuplet-cluster.
A quarter note can therefore be replaced by 7 septuplet notes that look like semiquavers.
You change a note into a septuplet-note (by 4/7) of its original duration with <alt-7>
