The ampersand-comma is a small 5-limit interval which functions as a unison-vector in many tunings. In particular, the ampersand-comma defines a promo which creates the 5-limit versions of miracle temperaments.
The size of the ampersand-comma may be described in the following ways:
2,3,5-monzo ratio ~ cents [-25 7, 6 > 34171875 / 33554432 31.56728925
The ampersand-comma was named by tuning-theorist Paul Rapoport, after his cat Ampersand.