My impression from seeing old documents is that Hangul was originally written vertically (top-to-bottom, in 'lines' that run right-to-left). Is this correct? If so, when did it start becoming more common to write Hangul horizontally left-to-right? Was there an obvious reason why there was a change?
Around the time Korea got colonized by Japan, the 한글 맞춤법 통일안 (Unified Hangul Grammar Rules) were written by the 조선어 학회 (Korean Language Society). The first appearance of something related to horizontal writing in Korean language history is found in their revision in the year 1940.
There was nothing that stated which direction was the 'default' one, but it did call for the usage of the following punctuation
" ", to be used in horizontally written sentences. Before then, only
『 』 - had been used in vertically written sentences.
Still, at that time, nobody really cared about writing horizontally. The noticeable changes in writing direction came after Korean modernization and their independence.
North Korea moved to a default of horizontal LTR formatting right after their government was established. The first North Korean grammar guidelines - 조선어 신철자법 (New Grammar Rules of Chosun Language) - were written in 1948. They state:
- 모든 文書는 왼쪽으로부터 오른쪽으로 橫書함으로써 原則을 삼는다.
Which translates to: All documents should be written horizontal, from left to right.
South Korea moved to the horizontal, LTR format slowly and gradually, compared to the North. This was pronounced in newspapers. 《한겨레신문》(The Hankyoreh), in 1988, was the first newspaper to use horizontal writing in their articles.
South Korean grammar guidelines - 한글 맞춤법 (Hangul Grammar) - never actually states that any direction is the 'default'. It's still okay to write both horizontally and vertically in the South, but since every Korean book and newspaper these days are written horizontally, horizontal writing is the 'de facto' format.
Wikipedia Links (All in Korean):
Unified Hangul Grammar Rules / https://ko.wikipedia.org/wiki/한글_맞춤법_통일안
Early to modern Korean grammar rules comparison / https://ko.wikisource.org/wiki/사용자:Nanuun-saram/한글_맞춤법_통일안_등_비교표
Horizontal and Vertical writing in East Asian Scripts (Korean) / https://ko.wikipedia.org/wiki/가로쓰기와_세로쓰기