How to split a pane horizontally in tmux

A horizontal pane split keeps one tmux task above another in the same window, which is useful when logs, test output, or an editor need to stay visible while a second shell keeps running below them.

In tmux, a top-and-bottom layout is created by the default C-b “ binding, which runs split-window -v. tmux calls this a vertical split because the panes are stacked vertically, even though the divider drawn across the window is horizontal.

The new pane opens below the current pane and becomes active immediately unless -d is used with split-window -v. The split fails when the current pane is already too small, and custom bindings in ~/.tmux.conf or a custom prefix can replace the default shortcut shown here.

Steps to split a pane horizontally in tmux:

Show pane numbers if the target pane is not obvious.
```
C-b q
```
The temporary overlay identifies the pane that will be split and lets one number key jump focus to it.
Press the default top-and-bottom split binding in the active pane.
```
C-b "
```
If tmux reports there is no space for another pane, enlarge the terminal or close an unused pane before splitting again.
List the panes to confirm the window now has an upper pane and a lower active pane.
```
$ tmux list-panes -F '#{pane_index}:#{pane_active}:#{pane_top}:#{pane_bottom}'
0:0:0:11
1:1:13:23
```
The fields are pane:active:top:bottom, so 1:1:13:23 confirms pane 1 is active in the lower half of the window.
Move back to the upper pane when work should continue there.
```
C-b Up
```
The command-prompt form C-b :split-window -v -d creates the lower pane without moving focus into it.