Ison is a drone note, or a slow-moving lower vocal part, used in Byzantine chant and some related musical traditions to accompany the melody, thus enriching the singing.
[3] Simon Karas is known to be interested in a double-ison technique, and he tried to reconstruct how it could sound like in the older 15th- 16th-century practices, when there appeared indeed some first attempts to create a "Native Byzantine alternative to Western polyphony".
The main reason for this gradual change obviously lies in the influence of Western music over Byzantine chanting practices.
[3] With this in mind, in most traditional Byzantine scores prior to the mid-20th century the ison was not even notated,[3] as it was assumed that to perform it is just too simple to bother to fix it in writing.
The ison is also supposed to be held across the gaps between the phrases, when the leading chanters, singing the melody, catch their breath.