/bloginteractive lessonsHow Git Really Works: Commits, Branches and Merges

How Git Really Works: Commits, Branches and Merges

Build a Git commit graph live in your browser: commit, branch, switch and merge, and see fast-forward vs merge commits. Finally understand what a branch and HEAD really are.

By Hamza GuetiouiNovember 3, 2026 8 min read 2 interactive

A glowing tree of branching and merging lines, like a subway map, on a dark background

TL;DR

Key takeaways

1A commit is a snapshot of your project plus a pointer to its parent commit(s).
2A branch is only a movable label pointing at one commit — creating one is instant and costs ~nothing.
3HEAD is “where you are now”: usually it points to a branch.
4If the target branch hasn't moved, git merge just fast-forwards the label; otherwise it creates a merge commit with two parents.
5Rebase rewrites your commits on top of another branch for a straight history — never rebase commits others already use.

Most people learn Git as a list of magic spells. But Git has a tiny, beautiful model underneath — and once you can see it, merge conflicts, detached HEADs and rebases stop being scary.

Commits are snapshots, chained together

Every commit stores a snapshot of your project, a message, an author, and a pointer to its parent commit. Follow parents backwards and you get history. The ID (like a1b2c3d) is a hash of all that content — change anything and you get a different commit.

Illustration of a chain of polaroid photos of a project, each pinned to the previous one with a string — Each commit is a photo of the whole project, linked to the one before.

The branch playground

Make a few commits, create a branch, commit on it, switch back to main, commit there too, then merge. Then reset and try merging without committing on main — watch for the fast-forward.

InteractiveGit graph sandbox

commit · branch · switch · merge

switch to:

$ git log --oneline

f1bbcd8 add homepage

Labels are branches. HEAD → shows where your next commit will go. A dot inside a circle marks a merge commit.

A branch is a sticky note

Did you notice? Creating a branch didn't copy anything. It just added a label. A branch is literally a tiny file containing one commit ID. When you commit, Git moves the label of the branch you're on forward — and only that one.

HEAD is the note that says which branch you're on. git switch feature moves HEAD and updates your files to that snapshot.

Branches are cheap. Commit often, branch freely — you can always move labels around.

Quick check

You create a branch 'feature' and make 3 commits on it. What happens to 'main'?

Fast-forward vs merge commit

Fast-forward

If main hasn't moved since you branched, there is nothing to combine. git merge feature just slides the main label forward. History stays a straight line.

Merge commit (three-way merge)

If both branches have new commits, Git compares both tips with their common ancestor and creates a new commit with two parents. If both sides changed the same lines differently, that's a merge conflict — Git asks you to choose.

terminal

git switch -c feature      # create + switch
git commit -am "add search" # label 'feature' moves
git switch main
git merge feature           # fast-forward or merge commit

And rebase?

git rebase main takes your branch's commits and replays them on top of the latest main, producing new commits with new IDs. The result is a straight history, as if you had started your work later.

A simple workflow that scales

Keep main always deployable and protected.
One short-lived branch per task: feat/…, fix/….
Open a pull request, let checks run, get a review, merge.
Delete the branch after merging — the commits live on in history.

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Frequently asked questions

What is a branch in Git?

A Git branch is a lightweight, movable pointer to a commit. When you commit on a branch, Git creates a new commit whose parent is the old tip and moves the branch pointer forward to it. That's why creating branches in Git is nearly instant.

What is HEAD in Git?

HEAD is a reference to what you currently have checked out. Normally it points to a branch name, which in turn points to a commit. If HEAD points directly at a commit instead of a branch, you are in a 'detached HEAD' state.

What is a fast-forward merge?

A fast-forward merge happens when the branch you are merging into has no new commits since the other branch was created. Git simply moves the branch pointer forward to the other branch's tip — no merge commit is needed.

Should I use git merge or git rebase?

Merge preserves the true history and is always safe on shared branches. Rebase creates a cleaner, linear history by replaying your commits on top of another branch, but it rewrites commit IDs, so only rebase local commits that nobody else has based work on.

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Written & animated by

Hamza Guetioui

Full-stack developer building fast, interactive websites and web apps with Next.js, React and TypeScript. I write these lessons the way I wish I had learned — by seeing things move.

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