Mongolian Ee and the Edges of an Alphabet

It is tempting to treat an alphabet as a fixed list, and Mongolian shows why that is a poor assumption. What a chart presents as thirty-five letters in a fixed order is really a core with a history of accretion around it. The core is the inheritance from Old Uyghur: the vowels that carry the harmony and the consonants of the native sound system, which is what was needed to write thirteenth century Mongolian and what the script was borrowed for. Around that core sit letters added for other purposes, and they are not all of the same kind. Some extend the vowels. Ten at the end of the alphabet supply consonants for words taken from other languages.

Beyond the alphabet proper there is a further apparatus of signs, encoded in Unicode under the name Ali Gali, for writing Sanskrit and Tibetan in full, with the distinctions those languages require and Mongolian does not make. And the same Unicode block holds the letters of Todo, Sibe and Manchu, three separate writing systems built on the same inherited shapes, which are not part of this alphabet at all and should not be counted into it. Knowing where the edges are matters. A count of the letters is only meaningful once you have said which of these layers you are counting.