Fast Forward vs Non-FF

When you merge one branch into another, Git has to decide what to do with the commits that have been added to the branch since the two paths last met. There are two outcomes, and which one you get depends on whether your branch has diverged from the one you are merging into.

Fast-forward happens when the branch you are merging has not moved since you branched off. There is nothing to reconcile, so Git simply slides the pointer forward and your commits become a straight continuation.

git checkout main
git merge feature

Non-fast-forward, also called a true merge, happens when both branches have new commits. Git has to combine two histories, and records the join as a merge commit with two parents.

git checkout main
git merge --no-ff feature

That merge commit is not optional bookkeeping, by the way. --no-ff forces one even when a fast-forward would have been possible, which is how teams record that a feature branch existed and was integrated, rather than leaving no trace of the branch at all.

Whichever you get, the result is the same tree of files. The difference is whether the history shows the join.

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