How do I open the previous file I was editing in a split window?

Explanation

In Vim, # is a special filename that always refers to the alternate file — the most recently active buffer before the current one. Passing # to :split (or :vsplit) opens that file in a new split without having to remember or type its name, letting you quickly set up side-by-side editing of two related files.

How it works

• # — expands to the name of the alternate file (shown as # in :ls)
• :split # — opens the alternate file in a horizontal split
• :vsplit # — opens it in a vertical split
• The alternate file is set whenever you switch buffers with <C-^>, :b, :e, or any other buffer command

Example

You are editing routes.py and switch to models.py. Now:

:vsplit #

Opens routes.py in a vertical split, so you can see both files at once without navigating away.

You can also use # in other Ex commands where a filename is expected:

:read #       " insert alternate file contents into current buffer
:diff #       " diff current file against the alternate file

Tips

• <C-^> (or <C-6>) switches between current and alternate file without a split
• :echo expand('#') prints the alternate file's full path — useful in mappings
• If no alternate file is set yet, Vim will report E23: No alternate file
• Use <C-w>s #<CR> for the same effect as :split # without leaving normal mode