Bar / measure
A metrically organized unit of time.
What it means
The bar (German Takt) is the metrically organized unit of time in music: a group of beats held together by a regular gradient of strong and weak beats. Every bar begins with the accented one; the following beats are lighter, with larger bars having additional secondary accents.
The word has several meanings that are to be kept apart. It designates, first, the unit between two barlines, second, the time signature, that is, the pattern of this unit, and third, colloquially, the beat itself (“to stay in time”). The bar differs from rhythm in that it supplies the constant framework of order, while rhythm is the changing distribution of the notes upon it. It differs from tempo in that it says nothing about speed.
In the notation
Bars are separated from one another by vertical barlines. The time signature stands as a fraction or as a sign at the beginning of the line and applies until a change. Bar counting begins with the first complete bar; an upbeat remains uncounted. In scores and parts, bar numbers stand at the beginning of lines or at regular intervals, so that everyone involved can communicate quickly.
Musical use
The consistent division into bars becomes established over the course of the sixteenth and seventeenth centuries; older music is notated without barlines and follows other organizing principles. Since then the bar has been the basic framework of almost all notated music. Composers work with it as much as against it: syncopations, shifts, and metre changes gain their effect precisely from the fact that a regular pattern is expected.
Other names
Misura is the Italian designation for bar. In performance instructions it is encountered, for instance, as senza misura (free in tempo) or a misura (strictly in time again).
Performance practice
Do not just count along but feel the gradient: a series of equally accented beats is not a bar. When practising, orient yourself by the metric accents, not by the barlines — the line is a reading aid, not an accent. Use bar numbers in rehearsal to name passages. And take care that an upbeat or a syncopation does not cancel the bar order: it remains in the background, even when the notes lie elsewhere.