Ask HN: Which direction do you consider a horizontal split, to be in?

So i just realized today that Vim and Tmux have different ideas about which direction a horizontal split is in.

Eg, vim puts a horizontal split to the bottom. Tmux puts a horizontal split to the side.

I've had a custom bind masking this quirky fact from me for years, and now it's bothering me haha. Is there a technically correct way to resolve the difference? Which direction is more correct?

Fwiw, iTerm2 splits to the bottom, like vim. Not sure about other editors.