git merge explained: fast-forward vs merge commit

1 Learn the idea

git merge <branch> brings the commits of another branch into the branch you are on. Depending on what the history looks like, git does one of two very different things.

Case 1: fast-forward

If your branch has no new commits since the other branch split off, git simply slides your branch pointer forward. No new commit is created — history stays a straight line.

Case 2: three-way merge

If both branches have new commits, git finds their merge base (the last common ancestor) and compares three snapshots: the base, yours, and theirs. Changes from both sides are combined into a new merge commit that has two parents.

2 Watch it happen

Fast-forward, then a real merge

Press Play or use → to step through. Watch the graph as each command runs.

terminal · simulated, no real repository is touched

Read this demo as text

```
$ git log --oneline --all
```
login has two new commits; main hasn't moved since login branched off. main is simply behind.
```
$ git merge login
```
Fast-forward! Git just slid main forward to login's commit. No merge commit was needed — there was nothing to combine.
```
$ git branch -d login
```
The work is in main now, so the login pointer is redundant. Deleting merged branches keeps the list tidy.

$ git switch -c cart
$ echo "cart logic" > cart.js
$ git add cart.js
$ git commit -m "Add cart"

New feature branch cart with one commit.

```
$ git switch main
$ echo "© 2026" > footer.html
$ git add footer.html
$ git commit -m "Add footer"
```
Meanwhile, someone commits directly on main. Now the two branches have diverged — each has a commit the other lacks.
```
$ git merge cart
```
No fast-forward is possible, so git finds the merge base, combines both sides, and creates a merge commit with two parents. Look for the double ring.
```
$ git log --oneline
```
History now contains both Add cart and Add footer, tied together by the merge commit.
```
$ git branch -d cart
```
Merged, so safe to delete. The commits live on inside main.

Fast-forward

Your branch had no new commits
Pointer just slides forward
No new commit, straight history

Merge commit

Both branches have new commits
Three-way combination via the merge base
New commit with two parents

git merge <branch>Bring <branch> into the current branch

git merge --no-ff <branch>Always create a merge commit

git log --oneline --allSee how branches relate before merging

git branch -d <branch>Clean up a branch after merging

3 Check your understanding

Question 1

You are on `main` and run `git merge feature`. What happens with this graph?

Question 2

And with this graph, on `main`, `git merge feature`?

Question 3

What is the "merge base"?

Question 4

You want to bring `feature` into `main`. Which is right?

4 Practice for real

Your mission

Land the feature

main and feature have diverged. Merge feature into main (creating a merge commit), then delete the feature branch.