Horizontal writing in the middle of vertical writing

Question

My character dictionary is written in vertical style. In this dictionary, columns of text are sometimes divided into two smaller columns, which are read right-then-left. I've read that this is called 割注{わりちゅう}. It looks like this:

This says りんれつ.

Sometimes, when there's only a single character in each column, it looks like horizontal writing: right-to-left. Based on this experience, I figured any horizontal writing in the middle of vertical writing would be read right-to-left.

However, on the first page of the novel 魔術{まじゅつ}はささやく, I came across this:

I think it says 24. But that would be left-to-right! My intuition tells me it should be so, because they are not Chinese or Japanese characters. But is this really correct? My experience with my dictionary tells me it might say 42 instead. Which is it?

More generally, how do I tell which way to read horizontal writing in the middle of vertical writing?

Answer 1

Numbers and sometimes short Latin alphabet words (like abbreviations) escape this rule. Even in 割注, I would always read numbers and Latin-alphabet words the "usual" way (which I think of as left-to-right and top-to-bottom, in this order).

I make sense of it like this:

For vertical writing it is top-to-bottom -> right-to-left.
For horizontal writing it is left-to-right -> top-to-bottom.

For horizontal writing, left-to-right comes first, so two characters should be typeset next to each other, read from left to right.

For vertical writing,top-to-bottom comes first, so two characters should be typeset one on top of the other first.

Breaking the column after a single character to type the next character in a separate column doesn't make much sense, so if text is typeset in a single line, it should be read left-to-right. If text is typeset in a single column, there is no confusion. In both vertical and horizontal writing, only the rule top-to-bottom applies.