If music is performed exactly as written, a dull, machine-like performance results. When performing music, musicians deviate slightly but significantly from what is nominally written in the score. Some of these deviations have been described in terms of context dependent rules.
For example, one rule states that in sequences of eighth notes, those in stressed
positions be played a little longer than those in unstressed position.
This rule is used both in jazz music and in some Baroque music, where the phenomenon was called "inégalles".
Here you can listen to the effects generated by this and other performance rules in a set of music excerpts.
Note that the sound is prerecorded in sound files/"coming directly from your CD player output"
The magnitude of the effect generated by a specific rule is determined by the rule quantity, k. If the quantity is high, the effect is great and salient.
Performance rules affect not only the duration of tones. Some rules alters loudness, change the exact pitch or the vibrato. Other rules create crescendos and diminuendos, change the tempo, or insert minute pauses between tones. The effects induced by a rule can be so subtle that they are very hard to perceive, or coarse, so that they catch the ear.
To realize how a rule affects the performance, it is often helpful first to compare a No Rule version of an example, where the rule is not applied, with an Exaggerated version, where the effect is great. There is also a Medium version, which we regard as musically preferable, and in some cases an Inverted, where the rule is producing the opposite effect, for instance shortenings instead of lengthenings.