In music a time point or timepoint (point in time) is "an instant, analogous to a geometrical point in space". Because it has no duration, it literally cannot be heard, but it may be used to represent "the point of initiation of a single pitch, the repetition of a pitch, or a pitch simultaneity", therefore the beginning of a sound, rather than its duration. It may also designate the release of a note or the point within a note at which something changes (such as dynamic level). Other terms often used in music theory and analysis are attack point and starting point. Milton Babbitt calls the distance from one time point, attack, or starting point to the next a time-point interval, independent of the durations of the sounding notes which may be either shorter than the time-point interval (resulting in a silence before the next time point), or longer (resulting in overlapping notes). Charles Wuorinen shortens this expression to just time interval. Other writers use the terms attack interval, or (translating the German Einsatzabstand), interval of entry, interval of entrance, or starting interval. Interonset interval The corresponding term used in acoustics and audio engineering to describe the initiation of a sound is onset, and the interonset interval or IOI is the time between the beginnings or attack points of successive events or notes, the interval between onsets, not including the duration of the events. A variant of this term is interval of onset. For example, two sixteenth notes separated by dotted eighth rest, would have the same interonset interval as between a quarter note and a sixteenth note: The concept is often useful for considering rhythms and meters. Time-point sets In serial music a time-point set, proposed in 1962 by Milton Babbitt, is a temporal order of pitches in a tone row which indicates the instants at which the notes start. This has certain advantages over a duration scale or row built from multiples of a unit, derived from Olivier Messiaen. For example, a measure may be divided into twelve metrical positions. In this equals sixteenth notes. The start of each position, or time point, may then be labeled, in order, 0–11. Pitches may then be assigned locations within measures according to their pitch set number, now their pitch/time-set number. In Babbitt's first example he shows subsequent numbers which ascend (0–11) as within the same measure (if four follows three