Split the current pane with a horizontal line to create a vertical layout
panes
// what it does
split-window -v stacks a new pane below the current one (a top/bottom split), matching the Ctrl+b " binding. -v is actually the default orientation, so plain split-window does the same thing. Focus moves to the new lower pane.
// shortcut
Ctrl + b"
// tmux command
: split-window -v// gotcha
Same directory caveat as -h: the new pane opens in tmux's start directory unless you pass -c "#{pane_current_path}". And -v (stacked) is the opposite of what "vertical layout" suggests to many users.