This form is a misspelling because it does not follow the sound conversion pattern (to which there are basically no exceptions), Middle Chinesey /j/ → Vietnamese <d>, as applied to every other Sino-Vietnamese syllable such as yæX → <dã>, ye → <di>, yuw → <du>, yek → <dịch>, yang → <dương>, etc. Thiều Chửu's dictionary also notes that <gia> is more of a popular spelling than a correct one, and lists it as a secondary spelling along with <da>. Nevertheless, <Gia Tô> is far more common than <Da Tô> and is even used in history textbooks published by Nhà xuất bản Giáo dục Việt Nam. This is perhaps because the syllable da is usually perceived as non-Sino-Vietnamese (it is homophonous with the native word meaning "skin"), its pronunciation is identical to that of gia in most modern accents, and gia is otherwise perceived as Sino-Vietnamese (in fact, it can only be found in given names and Sino-Vietnamese compounds such as gia vị, gia đình, Gia Định). Other words suffer this kind of common misspelling include *thiếu gia (thiếu da), *lão gia (lão da), *thái gia (thái da), etc.