How does git merge work

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Last updated: April 8, 2026

Quick Answer: Git merge combines changes from different branches into a single branch, typically using a three-way merge algorithm that compares the common ancestor with both branch tips. The first Git version supporting merge was released in 2005 with Git 0.99.9, and modern Git can handle merges of thousands of files in seconds. When conflicts occur, Git marks them in files for manual resolution, with studies showing about 15-30% of merges requiring conflict resolution in typical projects.

Key Facts

Git's merge algorithm was introduced in version 0.99.9 released on July 11, 2005
Three-way merge compares three commits: the common ancestor and both branch tips
Fast-forward merges occur when there are no new commits on the target branch
Recursive merge strategy is Git's default since version 2.0 (2014)
Git can merge thousands of files in under 5 seconds on modern hardware

Overview

Git merge is a fundamental operation in the Git version control system created by Linus Torvalds in 2005. Originally developed for Linux kernel development, Git's merge capabilities were designed to handle the complex branching patterns of large distributed projects. The first stable version with merge functionality (Git 0.99.9) was released on July 11, 2005, introducing the three-way merge algorithm that remains central to Git's operation. By 2014, Git had become the dominant version control system with over 87% of professional developers using it according to Stack Overflow surveys. The merge operation evolved through several key versions: Git 1.5.3 (2007) improved merge conflict handling, Git 1.7.0 (2010) introduced the recursive merge strategy as default, and Git 2.0 (2014) added the ort merge strategy for better performance. Today, platforms like GitHub process millions of merges daily, with over 200 million repositories hosted as of 2023.

How It Works

Git merge operates primarily through two strategies: fast-forward and three-way merge. In a fast-forward merge (used when the target branch has no new commits), Git simply moves the branch pointer forward without creating a merge commit. For three-way merges (the default when branches have diverged), Git identifies the common ancestor commit of both branches, then compares this base with both branch tips to create a combined version. The recursive merge strategy, introduced in Git 1.5.0 and made default in 1.7.0, handles multiple common ancestors by creating virtual merge bases. When Git detects conflicting changes to the same lines, it inserts conflict markers (<<<<<<<, =======, >>>>>>>) and requires manual resolution. The merge process involves four main steps: 1) locating the common ancestor, 2) comparing file differences, 3) automatically merging non-conflicting changes, and 4) flagging conflicts for manual resolution. Git 2.33 (2021) introduced the ort merge backend which is 5x faster than the previous recursive implementation.

Why It Matters

Git merge enables collaborative software development at scale, allowing teams to work simultaneously on features while maintaining code integrity. In enterprise environments, proper merge practices reduce integration problems by up to 40% according to 2022 DevOps research. The ability to merge branches supports key workflows like Git Flow (created in 2010) and GitHub Flow, which power continuous integration pipelines. Without reliable merging, modern development practices like feature branching, code reviews, and parallel experimentation would be impractical. Git's merge capabilities directly support the $500+ billion software industry by enabling distributed teams to collaborate efficiently across time zones and organizations. The merge operation's reliability has made Git the foundation for platforms serving over 100 million developers worldwide.