Cluster
Chord of seconds made up of closely spaced notes.
What it means
A cluster — English for a bunch or bundle — is a sonority made up of notes lying close together, usually at intervals of a second. Unlike an ordinary chord, whose notes are stacked in thirds and remain individually perceptible, the notes of a cluster merge into a surface of sound in which the individual pitches count for less than compass, density, and register.
In the notation
Clusters are notated in two ways. Either all the notes involved are written individually, which becomes confusing with a large extent; or a bar or a filled-in block is drawn between the lowest and the highest note, from which one gathers that all the notes in between are meant. Whether only white, only black, or all keys are meant is indicated on the piano by additional signs. The notation is not uniform, and editions usually explain their signs in the preface.
Musical use
As a deliberate means of sound, the cluster is a phenomenon of the twentieth century, in which tone colour became an independent parameter of shaping. On the piano it is struck with the flat of the hand, the fist, or the forearm; in the orchestra it arises through the distribution of closely neighbouring notes among many instruments and can grow and shrink through the gradual addition or removal of notes.
The cluster is to be distinguished from the chord with added notes, for instance the sixte ajoutée or the ninth chord: there a basic sonority stacked in thirds remains recognizable, to which individual notes are added; with the cluster this framework is missing.
The cluster was introduced in the twentieth century. Henry Cowell used it systematically and struck groups of keys with the fist or forearm; later it became a central material in the sound-mass music of the 1960s, for instance with György Ligeti and Krzysztof Penderecki. In the notation it usually appears as a bar between the lowest and the highest note.
Performance practice
With clusters, watch above all the limits — the lowest and the highest note — and the type of keys indicated; within this frame, the even touch counts. When striking with hand or arm, spare the instrument and yourself: weight instead of a blow. And listen to the surface of sound, not to individual notes.