Three explanations are held in the literature and none has carried the field. One derives the letters from the Greek minuscule cursive of the ninth century. One looks to Semitic and Caucasian models, among them Hebrew and Samaritan, Coptic, Armenian and Georgian. One holds that Constantine composed the signs from a small set of Christian figures, the cross, the circle and the triangle, so that the alphabet would belong to no existing script and would carry a Christian sense in its shapes. This is a genuine disagreement rather than a gap: camps exist and each can be read. What none of them has is a witness close in date to the alphabet, since the earliest Glagolitic manuscripts are a century or more later than the mission and were produced in a different country.
What would resolve this A securely dated ninth century Glagolitic witness, or the recovery of one from the under-text of a palimpsest by multispectral imaging, would give the shapes a form early enough to be compared against the proposed models rather than against their tenth and eleventh century descendants. Failing that, a systematic morphological comparison of the whole Glagolitic repertoire against dated Greek minuscule hands and against the Semitic and Caucasian scripts proposed, published with the negative results as well as the positive ones, would establish how much of the resemblance survives measurement.
OpenSlavic palaeography; Greek palaeography; comparative script studies; multispectral imaging of palimpsestsPeter T. Daniels and William Bright (eds.), The World's Writing Systems