Agogics
Small tempo changes that enliven the performance.
What it means
Agogics denotes the small tempo changes with which a performance is shaped: the slight stretching of a target note, the barely perceptible pressing in an intensification, the yielding at the end of a phrase. What is meant are deviations from the strict pulse so slight that they are perceived not as changes of tempo but as liveliness.
The term was introduced into music theory in the late nineteenth century as a counterpart to dynamics: the latter concerns the gradation of loudness, agogics that of time.
There is an overlap with tempo rubato. Rubato means an expressly free shaping, frequently with larger deviations; agogics is the umbrella term for all temporal subtleties, including the inconspicuous ones that belong to every living performance.
In the notation
Agogics is as a rule not notated. It belongs to what the performer brings to the music. Notated markings such as ritardando, accelerando or tenuto denote larger or targeted deviations; the fine ones in between arise from the phrasing, the harmony and the texture.
Musical use
Every living performance works with agogic means, even in music that seems strict in time. Dissonances, the high points of a line and changes of harmony call for minimal delays so that they become audible. How far such freedoms may go depends on the style: in dance-like music the latitude is small, in Romantic piano music large.
The term was coined by Hugo Riemann in 1884 as a counterpart to dynamics. Agogics encompasses rubato, small stretchings at phrase endings and the lingering on important notes; it is usually not notated but a matter of interpretation.
Performance practice
Use agogic means in a targeted way and not continuously; a performance that stretches constantly loses its pulse. Take and give in balance, so that the basic tempo is preserved. Orient yourself by the harmony: where a tension arises, there may be time. And take yourself back to the metronome now and then when practising, as a check to see whether shaping has imperceptibly become dragging.