Why is Japanese written vertically?

Japanese can be written both vertically (縦書き, tategaki) and horizontally (横書き, yokogaki), formal Japanese writing is usually written vertically with lines moving from right to left. The Japanese writing system takes its roots from the Chinese writing system, and as such was initially oriented vertically. 横書き was introduced later with the introduction of western language dictionaries, the reason being because having both vertical and horizontal text in one book was rather impractical and made it more difficult to use.