Some writing systems go right to left, such as Arabic:
Others left to right, for example modern romance languages:
Languages like Japanese traditionally used vertical writing systems where the columns proceed from right to left:
Are there known reasons why these writing systems developed in this way? Does it perhaps have to do with the mediums in which the languages were written or read (for example, on scrolls, tablets, clay)?
Do we know?
Also, are there languages which go from bottom to top, for example, or in a different format altogether?