Macro join_pane 

macro_rules! join_pane {
    (@cmd ($cmd:expr) -b, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -d, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -f, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -h, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -v, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -l $size:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -s $src_pane:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -t $dst_pane:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr)) => { ... };
    () => { ... };
    (($cmd:expr), $($tail:tt)*) => { ... };
    ($($tail:tt)*) => { ... };
}

Like split-window, but instead of splitting dst-pane and creating a new pane, split it and move src-pane into the space

Manual

tmux ^3.1:

join-pane [-bdfhv] [-l size] [-s src-pane] [-t dst-pane]
(alias: joinp)

tmux ^1.7:

join-pane [-bdhv] [-l size | -p percentage] [-s src-pane] [-t dst-pane]
(alias: joinp)

tmux ^1.2:

join-pane [-dhv] [-l size | -p percentage] [-s src-pane] [-t dst-pane]
(alias: joinp)