' splits across the pane, and it’s on the middle row if you ignore the function keys. % splits the pane down the middle, and it’s located in the middleish of the keyboard, almost splitting the two halves from each other. So if it’s not visual, what’s the mnemonic for these defaults? % kind of looks like two panes side by side, though it could also just be two panes stacked ' would definitely be two panes side-by-side, though. These make more sense at first glance: | and _ look more like the end result more than % and ' do. It seems a common remapping is to change these to Ctrl-b | for vertical, and Ctrl-b - (or Ctrl-b _, or both) for horizontal. A horizontal split (one pane on top, one pane on the bottom) is Ctrl-b '. I could turn it back by resizing it to full and minimize again.

After Cmd + D, the terminal screen turns black. While trying to fall asleep, I ended up thinking about the key combinations used to create splits inīy default, you use Ctrl-b % to create a vertical split (i.e. Terminal vertical split Hi, I try to split the terminal window into two tabs.

Tmux vertical split