That's a good one 🕊️

So they come together like this:

topdir
   |
subdir

Also, with a little imagination, if you interpret it as a fraction, you'd have the topdir as a common denominator like this:

topdir\subdir1, topdir\subdir2

You would also have to read it from right to left for it to make sense though:

subdir1/topdir, subdir2/topdir

But considering this approach, when writing the parts of the fraction below each other, the topdir is below the subdir (like a real tree growing from the ground lol). The "subdir is under topdir" analogy gets lost, which is my main problem with the backslash approach:

topdir\subdir

=>

subdir
***
topdir