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In page Old Turkic script:

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Old Turkic being a synharmonic language, a number of consonant signs are divided into two "synharmonic sets", one for front vowels and the other for back vowels. Such vowels can be taken as intrinsic to the consonant sign, giving the Old Turkic alphabet an aspect of an abugida script. In these cases, it is customary to use superscript numerals ¹ and ² to mark consonant signs used with back and front vowels, respectively. This convention was introduced by Thomsen (1893), and followed by Gabain (1941), Malov (1951) and Tekin (1968).[citation needed]