dialectal differences have generally distinguished between three types: Phonological: the phonemic inventories and phonotactic restrictions Phonetic: how a given phoneme is realized phonetically (RP and Australian English, for example, have almost the same exact phoneme system but with notably different realizations of the vowels). This distinction covers differences in the range of allophonic variation. Incidence: one phoneme rather than another occurs in a given word or group of words (such as grass, which has the same vowel of farce in RP but not in GA.) Wells expanded on this by splitting up the phonological category into "systemic" differences (those of inventory) and "structural" differences (those of phonotactics). In addition, both Wells and Weinreich mention realizational overlap, wherein the same phone (or a nearly identical one) corresponds to different phonemes, depending on accent. Some examples: Autistic in Canadian English overlaps with the way speakers of Received Pronunciation say artistic: Impossible in General American overlaps with RP impassable: notes a similar phenomenon in Western Pennsylvania, where occurs either as the vowel of ashes or as the vowel of tiger but no speaker merges the two vowels (i.e. a speaker who says will not say ). Realizational overlap occurs between the three dialects of Huastec, which have the same phonological system even though cognate words often do not have the same reflexes of this system. For example, while the Central and Potosino dialects both have ch and ts-type sounds, the words they are found in are reversed: Yuen Ren Chao created a diaphonemic transcription of major Chinese varieties, in both Latin and Chinese character versions, called "General Chinese". It originally (1927) covered the various Wu dialects, but by 1983 had expanded to cover the major dialects of Mandarin, Yue, Hakka, and Min as well. Apart from a few irregularities, GC can be read equally well in any of those dialects, and several others besides. Qur'anic Arabic uses a diaphonemic writing system that indicates both the pronunciation in Mecca, the western dialect the Qur'an was written in, and that of eastern Arabia, the prestige dialect of pre-Islamic poetry. For example, final was pronounced something like in Mecca, and written ي , while it had merged with in eastern Arabia and was written as ا . In order to accommodate both pronunciations, the basic letter of Meccan Arabic was used, but the diacritic was dropped: ى. Similarly, the glottal stop had been lost in Meccan