Japanese was written vertically, so characters are ordered from top to bottom while it is the lines which are separated from left to right. The slow leftward advancement allows a person writing on a scroll to write with their right hand while unrolling the scroll gradually with their left. In modern times you can write Japanese horizontally, but in that case you usually write from left to right. Characters written horizontally from right to left is only done in exceptional cases.
I just realized that top-to-bottom also solves the problem with smearing discussed in another comment. Your hand is moving down, away from the fresh characters. By the time you move over to the next line, the top is already dried. (Especially with the time it takes to fill a line with kanji, which are denser than Latin script.)
Japanese was written vertically, so characters are ordered from top to bottom while it is the lines which are separated from left to right. The slow leftward advancement allows a person writing on a scroll to write with their right hand while unrolling the scroll gradually with their left. In modern times you can write Japanese horizontally, but in that case you usually write from left to right. Characters written horizontally from right to left is only done in exceptional cases.
I just realized that top-to-bottom also solves the problem with smearing discussed in another comment. Your hand is moving down, away from the fresh characters. By the time you move over to the next line, the top is already dried. (Especially with the time it takes to fill a line with kanji, which are denser than Latin script.)
Thanks, that makes a lot of sense.
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