concept in ancient Greek music, pertaining to the placement of scale patterns at different pitches and closely connected with the notion of octave species (q.v.). Through transposition of the Greater Perfect System (comprising two octaves descending from the A above middle C to the second A below) to a higher or lower pitch level, each tonos causes a different octave species to fall within the octave e′–e (E above middle C to the E below), which is important in Greek performance. The names of the tonoi correspond to the names of the octave species that are created between e′ and e when the tonoi are used. In effect, a tonos can cause the octave species bearing its name to fall within the e′–e octave. For example, when the Greater Perfect System is begun on the pitch b′ (rather than a′, as in the abstract), the octave species falling between e′ and e is the Phrygian; hence, the tonos is also Phrygian. According to most modern scholars, the tonoi thus render the highly theoretical Greater Perfect System eminently practical in actual performance.

The concept of tonos first appeared in the 4th century BC and became the subject of controversy almost immediately. Aristoxenus (flourished 4th century BC) lists 13 tonoi; Ptolemy of Alexandria (2nd century AD), 7; other theorists, 15. The conflicting views of the Greek theorists have their modern counterparts. For example, some scholars believe the tonoi were real keys in the modern sense—i.e., that they provided contrasting tonal centres of specific pitch (although the Greeks did not have an absolute pitch standard). Others insist that the tonoi were abstract theoretical concepts or that they were melodic frameworks (melody types).