The origins of Thaana are unique among the world's writing systems: The first nine letters (h–v) are derived from the Arabic numerals, whereas the next nine (m–d) were the local Indic numerals. (See Hindu-Arabic numerals.) The remaining letters for loanwords (z–ch) and Arabic transliteration are derived from phonetically similar native consonants by means of diacritics, with the exception of y, which is of unknown origin. This means that Thaana is one of the few writing systems not derived graphically from the original Semitic alphabet—unless the Indic numerals were (see Brahmi numerals). (The Ogham script of Ireland is another example, which also has some relation to numbers, since most of its letters are differentiated from others in a way similar to tally marks.)