A facility built into some synths, mostly digital synthesizers, which allows the performer to set up a keyboard temperment in which certain notes in each octave may be sharp or flat relative to the equal tempered tuning to which non-electronic keyboard instruments are normally tuned. One advantage of synthesizers compared to e.g., pianos or guitars, is the ease by which different keyboard temperments may be set up and changed during performance.
An example of a non-equal-tempered tuning sometimes used in electronic music is just intonation. The twelve-tone Western music scale is based on a mathematical construct called the "circle of fifths", which calculates all of the note values starting from a base note. It produces intervals relative to that base note which are more mathematically pure, especially in the important intervals such as the major second, major fourth, and perfect fifth. The main problem with just intonation is that, to gain the advantages, the music being played must be in the key of the base note; if it is not, some intervals will be far out of tune. Because of this, just intonation must have the base note specified; for instance, if A was used as the base note to calculate the scale, that is referred to as "just A". Often, synths which contain alternate tuning tables will have a pre-built table for just C, even though some other base note (B, D, or E) would work better for a lot of electronic music.