Arabic is a cursive script, which means that most letters are connected, similar to handwritten English. This makes it somewhat awkward to write Arabic vertically. It’s actually pretty rare to see vertical Arabic writing, except for the occasional billboard, shop sign or in crossword puzzles.
Writing Arabic vertically, with disconnected letters is not wrong in and of itself, but is certainly not as aesthetically pleasing as Arabic script written horizontally, with connected letters.
What if, however, you are set on writing vertical Arabic, e.g. for a tattoo or a design?
There are three ways in which Arabic can be written vertically:
1. Write letters in their isolated form underneath each other
This solution is perhaps the least favorable one, as it will result in disjointed letters, thereby destroying a good part of what makes the the Arabic script so appealing.
2. Write words underneath one another
This is quite a common way to fit Arabic text into a vertical space, preserving the connected nature of the letters.
Of course, in some situations, this solution might still take up too much space horizontally.
3. Rotate the whole text counter-clockwise by 45°
This solution preserves the cursive character of the script with the letters being connected, while allowing you to fit the writing into a vertical area.
The Omniglot website presents an interesting script called Sharjastani which is Arabic script adapted to vertical writing. It looks promising, but I doubt that it will be widely adopted. Here is a line of Arabic written using this script:
image credits: ominiglot.com
As this is essentially a fantasy script, I wouldn’t recommend its usage where readibility is crucial.
What is the best way to write Arabic vertically? I would say that the third option is the best insofar as it preserves the beauty of the script and still allows for flexibility in terms of placement of the text.