Q. Which do you commonly use to write Japanese: vertical or horizontal writing?
A. In old days Japanese was only written vertically. All historical documents are written in this style. However, with the introduction of western materials, the alphabet, Arabic number and mathematical formulas, it was not as convenient to write vertically.
Since each kanji and kana has its own meaning, they could be arranged either way without losing their meanings. Science-related text, which include many foreign words, gradually had to be changed to horizontal text.
Today most school textbooks, except Japanese or classical literature, are written horizontally. Young people mostly write this way, though some older people still prefer to write vertically as it looks more formal sometimes. Most general books are set in vertical text. The Japanese don't have problem to read or write either way. However it could be said that the horizontal style is more common nowadays.
Vol. 40: How do I put Japanese characters together for a tattoo?