In compressed tuning, also known as narrowed tuning, two notes an equivalence apart, whose fundamental frequencies theoretically have an exact ratio, are tuned slightly closer together (a compressed or narrowed equivalence).
Stretched tuning is used even outside of a xenharmonic context. Most acoustic and some electric pianos have overtones which do not exactly line up with the harmonic series, so stretched octaves are usually used to compensate.
In xenharmonic music
Within a xenharmonic context, stretched or compressed tuning may be used to reduce the harmonic entropy of a scale without sacrificing its melodic shape, or to achieve other artistic goals.
Examples include (but are not limited to):