<Tr> <Th> high iotizing </Th> <Td> ᅥ ə </Td> <Td> </Td> <Td> ᅮ u </Td> </Tr> <Tr> <Th> low iotizing </Th> <Td> ᅡ a </Td> <Td> </Td> <Td> ᅩ o </Td> </Tr> <P> There was presumably a third parameter in designing the vowel letters, not mentioned in the Haerye, namely choosing horizontal ᅳ ɨ as the graphic base of "closed" (rounded) ᅮ u and ᅩ o, and vertical ᅵ i as the base of "open" (unrounded) ᅥ ə and ᅡ a . The horizontal letters ᅳ ᅮᅩ ɨ uo represented back vowels * (ɯ), * (u), * (o) in the fifteenth century, as they do today, whereas the fifteenth - century sound values of ᅵ ᅥᅡ i ə a are uncertain . Some linguists reconstruct them as * (i), * (ɤ), * (e), respectively (and reconstruct obsolete ᆞ ʌ as * (a)); others as * (i), * (e), * (a) (with ᆞ ʌ as * (ʌ)). In the latter case, the vertical letters would have represented front vowels, the dot the sole central vowel, and the vowel harmony, described as "shallow" vs "deep", would have been one of vowel height, with the yang vowels lower than their yin counterparts . </P> <P> A resemblance of' Phags - pa ꡠ e to Hangul ᅳ ɨ (both horizontal lines), and of' Phags - pa ꡡ o to Hangul ᅩ o (both horizontal lines with an upper point in the middle), would back up Ledyard's theory if a connection were proven . </P>

When was hangul created and how many years has it been in operation