The organization of time in music is just as fundamental (if not more so) than the system of pitches. Melodies are shaped just as much by their rhythms as they are by their melodic contours, and harmony depends on certain pitches being played synchronously. Many percussion instruments don't produce definite pitches at all, making them solely rhythmic instruments. The foundation of all rhythm is the beat.
The steady pulse, or beat, provides the framework for all rhythm in music. We dance to the beat, a conductor keeps the beat for an orchestra, and a metronome clicks the beat. The vast majority of music has a beat, even if sometimes it is not easy to follow.
|grave||solemn (very, very slow)||~ 40|
|largo||broad (very slow)||~ 50|
|presto||very fast||~ 180|
The tempo is the speed of the beat. A tempo may be designated with general terms such as "fast" or "slow," or it can be measured precisely in beats per minute (bpm). The second-hand of a clock ticks at 60bpm, which is a good reference point.
It is common to find tempo markings in Italian (very fast: presto, slow: lento) or another language such as French or German. Although some terms can be understood by consulting a standard bilingual dictionary, other terms are conventional rather than literal. For example, allegro literally means 'happy' or 'cheerful' in Italian, but in music allegro means fast.
Rubato - "Stealing" Time
Measuring tempo in beats per minute can seem very rigid, but sometimes a soloist is permitted to slow down and speed up the tempo for expressive purposes. This is called rubato or tempo rubato from the Italian for 'stolen time'. This often happens in a cadenza - an extended solo section in a concerto. For an example of a cadenza performed with rubato, listen to Mozart's Piano Concert No. 20 in D minor (K.466) with a cadenza written by Beethoven (playback will begin just a few seconds before the cadenza).