Split the current pane with a vertical line to create a horizontal layout
panes
// what it does
split-window -h splits the active pane in two side by side (a left and a right pane), the same result as the Ctrl+b % binding. The -h flag divides the panes with a vertical line, so despite the letter it produces a horizontal row of panes rather than stacked ones. The new pane runs your default shell and becomes the active pane.
// shortcut
Ctrl + b%
// tmux command
: split-window -h// gotcha
The -h/-v naming is inverted from most people's intuition: -h gives a left/right split, not top/bottom. The new pane also starts in tmux's default directory, not the current pane's — add -c "#{pane_current_path}" to inherit the working directory.