Git: When to use three dots vs two

I endlessly misremember when to use '...' in git versus '..'. That ends today:

To see the commits or diffs introduced by a branch (the filled circles ●):

    +●  m           git log m..f
     |  ○
    +●  |           git diff m...f
      \ ○

To see the commits or diffs between the tip of one branch and another:

     f              git log m...f
    +●  m           All commits look the same,
     |  ●-          unless you use --left-right, which
    +●  |           shows where each commit comes from.
      \ ●-
       \|           git diff m..f
        ○           '-' commits are shown inverted,
                     i.e. additions as deletions.

To see the commits from 'f' back to the beginning of time:

    +●  m           git log f
     |  ○
    +●  |           (diffs back to start of time are just
      \ ○            the contents of the working tree)

Throughout, omitted branchname defaults to current HEAD, i.e, the one of the above that you almost always want:

git diff m...f

is the same as

git checkout f
git diff m...


git checkout m
git diff ...f

Is there a word for unicode ascii art?