Split horizontally or vertically – which one is which?

Question

Given some object, you can split it with a horizontal cut into two objects that are laid out vertically (above each other), or you can split it with a vertical cut into two objects that are laid out horizontally (next to each other).

When you say "split horizontally" or "split vertically", which one is which?

Two pairs of examples from Unix/Linux systems:

The two probably most popular text editors (emacs and vim) disagree. Placing (split-window-horizontally) in .emacs, or vim's -O[N] Like -o but split vertically options achieve the same layout: two panes next to each other.

The two probably most popular solutions for having many terminals neatly arranged: tmux and terminator also disagree, the side-by-side layout is achieved in tmux by tmux splitw -h (quoting the manual: "Windows may be split horizontally (with the -h flag)"), and in terminator by remotinator vsplit or the "Split Vertically" menu entry.

My question is meant to be a generic one, not restricted to the computer world.

Centaurus · Answer

When you split some object horizontally you cut it across its long axis.  To cut it vertically you section it through its short axis. That's the way I understand it.

This is a horizontal split.

Stu Cox · Answer

'Horizontal' means 'relating to the horizon', so strictly speaking whether a split is vertical or horizontal depends on its orientation relative to the ground. Or less strictly, 'horizontal' is whatever the observer considers to be left/right rather than up/down.
Whether that's the long or short axis has no bearing (which is good for the OP's terminal window example, otherwise terminology would need to change based on the window's proportions!)
I do agree with @Centaurus that the direction of the split is the important thing though (like if there is a split in a piece of wood).
Horizontal split = the dividing line goes from left to right, and remaining parts are stacked on top of one another.
But it's clearly not always entirely obvious to a reader, so it's worth trying to find another way to communicate the intended orientation where possible.

Split horizontally or vertically – which one is which?

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