Hanja
Although a phonetic Korean alphabet, now known as Chosŏn'gŭl or Hangul, had been created by Sejong the Great, it did not come into widespread official use until the late 19th and early 20th century. Thus, until that time it was necessary to be fluent in reading and writing Hanja in order to be literate in Korean, as the vast majority of Korean literature and most other Korean documents were written in Literary Chinese, using Hanja as its primary script. Today, a good working knowledge of Chinese characters is still important for anyone who wishes to study older texts (up to about the 1990s), or anyone who wishes to read scholarly texts in the humanities. Learning a certain number of Hanja is very helpful for understanding the etymology of Sino-Korean words, and for enlarging one's Korean vocabulary. Today, Hanja are not used to write native Korean words, which are always rendered in Hangul, and even words of Chinese origin—"Hanja-eo" ()—are written with the Hangul alphabet most of the time, with the corresponding Chinese character often written next to it in order to prevent confusion with other characters or words with the same phonetics.
Jeju language
This vowel is not entirely obsolete outside of Jeju, as it can be found in various brand names. It is often just an aesthetic replacement for the ㅏ vowel in standard Korean.A notable difference between Jeju and the dialects of mainland Korea is a lack of speech formality or honorific deference. For example, while a speaker of the Gyeonggi dialect might say "annyeong haseyo" ("Hello") or "ban'gapseumnida" ("Pleasure to meet you") to an older person, a speaker of the Jeju dialect would say "ban'gapsuda", which would be roughly equivalent to "Howdy" or "Nice ta meet ya" in Gyeonggi dialect. In mainland Korea, it would be inappropriate for a child to say this to an adult, but this usage is normative in Jeju. This is also what caused a further division of language between Jeju island and mainland Korea; Jejueo was dubbed inferior to the standard Korean because of its casual tone and lack of the formal morphemes such as 요 (yo) in 안녕하세요 (an-nyong ha-sae-yo "hello").