It is my understanding that くの字点{じてん} (seen above) is the "double or multiple repeat marker" in vertical writing. Given that it can be used to repeat two or more syllables, how do you know how many preceding syllables to repeat? Is it just a matter of memorization?
For example (using(く)to represent the symbol in horizontal writing below):
ひとり(く)= ひとりひとり is easy to understand because it is a word I am already familiar with.
However, if I read 「くり返し(く)」and wasn't familiar with the word for repeatedly, I may incorrectly read it as 「くり返し返し」 and not 「くり返しくり返し」. I realize this is a stilted example, but I'd like to nip any possibility of confusion down the line by understanding the mark completely from the start.