Syriac Christianity split along the christological arguments of the fifth century, and its chant split with it. The West Syriac churches, which today include the Syriac Orthodox, the Syriac Catholic, the Maronite and the Syro-Malankara, order their melodies in the Beth Gazo, the treasury of chants, a repertory of melody models distributed across eight modes that rotate week by week through the year. The East Syriac churches, the Assyrian Church of the East, the Chaldean Catholic Church and the Syro-Malabar Church, order their office through the Hudra, the cycle, with its own repertory and its own way of assigning tunes.
Calling these two dialects of a single tradition understates the distance. They share a language, a founding poet and a set of forms, and they diverge in the melodies themselves, in the assignment of modes and in the shape of the office those melodies fill. Someone trained in one does not simply sing the other, which is why the atlas records the churches on either side of the divide as holding the same tradition differently rather than as holding the same thing.
The geography is wider than the name suggests. The Church of the East carried its chant along the trade routes as far as Central Asia and China, and its surviving eastern extension is on the Malabar coast of India, where East Syriac chant is still sung in Kerala. The proportion actually sung in Syriac now varies sharply: some communities keep it almost entirely, while others have moved much of the office into Arabic, Malayalam or English, keeping the melodies and changing the words they carry.