What was the exact letter inventory and order of the alphabet Constantine-Cyril devised in 863?

Why it is open. No manuscript survives from the Moravian mission itself. The oldest Glagolitic codices are tenth and eleventh century copies made a century or more after the alphabet was made, and they differ among themselves in which letters they carry and in what order they list them. Scholars do not hold opposing positions on the original inventory, because there is nothing to hold a position against: the question is left open for want of a ninth century witness rather than argued between camps. The letters this atlas records for Glagolitic follow the order and inventory of the surviving tradition as the Unicode Standard arranges it, which is the best available reconstruction and not the ninth century original.

What would resolve it. A ninth century Glagolitic witness, whether a codex, a fragment, a palimpsest under-text or a dated inscription, carrying an abecedary or enough continuous text to establish which letters were in use during the mission itself. Palimpsest recovery by multispectral imaging of the surviving early codices is the most likely route, since several of them are known to overlie earlier writing.

Field. Slavic palaeography; codicology; multispectral imaging of palimpsests