Is this true? Are there other writing systems that are purely vertical?
ETA: To clarify, in CJK writing systems, it's the individual characters that are zigzagging, not the lines themselves. For instance, if you were to write 想, you would write 木, then 目, then 心, with the first two components written horizontally before you continue down vertically to the 心. Whereas in Mongolian, there's no horizontal movement before you write downward, from what I can gather from the writing.