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I assume the LOCAL and REMOTE files are just what their name suggests, but what are BASE and BACKUP for?

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Git performs a three-way merge, finding the common ancestor (aka "merge base") of the two branches you are merging. When you invoke git mergetool on a conflict, it will produce these files suitable for feeding into a typical 3-way merge tool. Thus:

  • foo.LOCAL: the "ours" side of the conflict - ie, your branch (HEAD) that will contain the results of the merge
  • foo.REMOTE: the "theirs" side of the conflict - the branch you are merging into HEAD
  • foo.BASE: the common ancestor. useful for feeding into a three-way merge tool
  • foo.BACKUP: the contents of file before invoking the merge tool, will be kept on the filesystem if mergetool.keepBackup = true.
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No. LOCAL is the version in HEAD. BACKUP was the version that was on-disk before you invoked mergetool. It probably contains the diff3 conflict markers and you may have edited it before invoking mergetool.
See here also : stackoverflow.com/questions/11133290/git-merging-using-meld
here is a good article explaining merging cases including the three-way merging : git-scm.com/book/en/v2/…
the words LOCAL/REMOTE is somehow misleading, makes me thinking they are referring to "my changes"/"remote changes", but actually usually in context of "merge into master", LOCAL is the target branch which is other's modification, and REMOTE is the source branch which is my modifications. :)
They are... Are you doing a rebase? Because the sides are reversed in a rebase.
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(UPDATED) Clarification:

  • What REMOTE and LOCAL refers to depends on the git command you are executing.

Two examples for git merge and git rebase:

  • LOCAL refers to the internally checked out branch by git, while performing your command.
    • Meaning in git checkout A; merge B ➜ A = LOCAL, B = REMOTE
    • Meaning in git checkout A; rebase B ➜ A = REMOTE, B = LOCAL

BASE is always the the origin of both files ('without any modifications') - independent on the command.

Git three way merge examples

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Upvote for using a graphic. The reason this stuff is hard to understand is that most merge tools use only text. And, CLI is no substitute for a chart to make GIT make sense. I wish I could upvote more times. (Some people need to embrace that CLI is one of many tools and not the only one, and sometimes it's the wrong tool.)
The graphic also made this make more sense for when doing merges in my own local repository. "Local" and "remote" are confusing names if the "remote" commit is actually a commit within my "local" repository. Too many names have multiple meanings in git.
Did something change? I've tested this, and the LOCAL file contains "done by me", and the REMOTE contains "done by others". Both in TortoiseGit and in git mergetool.
In accepted answer local is considered as 'ours' side? Can someone explain diferences in acepted answer?
REMOTE is mine?!! that is confusing!
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According to https://git-scm.com/docs/git-mergetool

When git mergetool is invoked with this tool (either through the -t or --tool option or the merge.tool configuration variable) the configured command line will be invoked with $BASE set to the name of a temporary file containing the common base for the merge, if available; $LOCAL set to the name of a temporary file containing the contents of the file on the current branch; $REMOTE set to the name of a temporary file containing the contents of the file to be merged, and $MERGED set to the name of the file to which the merge tool should write the result of the merge resolution.

However, there seems to be a difference between a rebase command and a merge command.

Merge uses your local branch as LOCAL and the branch you're merging in as REMOTE

Rebase uses your local branch as REMOTE and the branch you're rebasing onto as LOCAL

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I agree with @Shanakor ,one of the the answer is wrong. The so called local or remote basically indicates what git does in terms of “check out/switch branch” . If you are currently at branch A, and if you run git merge B, git stays in your Branch A for merge command, so local is A; if you are currently in Branch A, and you run git rebase B, (or rebase --onto B), internally, git check out /switch to B, and then the local is of course B;

See: https://git-scm.com/docs/git-rebase

Quote: If is specified, git rebase will perform an automatic git switch before doing anything else. Otherwise it remains on the current branch.

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