A long time ago I learned about the gf command in Vim.
It goes to the file whose name is under the cursor.
This can be pretty handy if you have a bunch of Markdown files that reference each-other.

But I never knew to go back to the previous file again.
So this week I took the time to find out how to do it.
And it turns out that there are multiple ways :-)

The most straightforward one is: CTRL-6
And there is also CTRL-o (back to the 'outer' file) with its companion CTRL-i (go forward to the 'inner' file).
CTRL-o and CTRL-i can be used to jump back and forth on the list of files opened via gf.