Is vertical writing from right to left or left to right?

From left to right. Because in ancient times our writing materials were bamboo slips, one by one, long and thin, which determined that our writing was vertical. And because the bamboo slips are mostly a volume, most people are right-handed pen, so the left hand to support the bamboo slips, just because from right to left is the left hand of the hand, so in order to facilitate the order of writing at that time is also from right to left.