Three-Way Merge: When Branches Truly Diverge

git merge-base a b

Finds the real, most recent commit both a and b share as an ancestor — the actual starting point git uses for a three-way merge. Confirmed directly in this lab: it returned the exact real merge commit both branches had genuinely diverged from.

Three-way merge

A merge where git compares three real inputs — the common ancestor, and each side's current tip — rather than simply moving a pointer. Confirmed directly in this lab combining two genuinely independent files (spice-rack.md and freezer-labels.md) with zero conflict, since neither side touched what the other changed.

🌱 Create Real, Independent Work on Both Sides

Give the feature branch its own commits, and let master gain a genuinely separate one.

cd ~/team-recipes && git switch -c feature/spice-rack

cd ~/team-recipes && printf 'Spice Rack\n\nOrganize spices alphabetically for quick access.\n' > spice-rack.md

cd ~/team-recipes && git add spice-rack.md

cd ~/team-recipes && git commit -m 'Add spice rack organization guide'

cd ~/team-recipes && printf '\nReplace ground spices every 12 months for best flavor.\n' >> spice-rack.md

cd ~/team-recipes && git add spice-rack.md

cd ~/team-recipes && git commit -m 'Add spice freshness guidance'

cd ~/team-recipes && git push -u origin feature/spice-rack

student@lab:~$ cd ~/team-recipes && git switch -c feature/spice-rack Switched to a new branch 'feature/spice-rack' student@lab:~$ cd ~/team-recipes && printf 'Spice Rack\n\nOrganize spices alphabetically for quick access.\n' > spice-rack.md student@lab:~$ cd ~/team-recipes && git add spice-rack.md student@lab:~$ cd ~/team-recipes && git commit -m 'Add spice rack organization guide' [feature/spice-rack 6e16df9] Add spice rack organization guide 1 file changed, 3 insertions(+) create mode 100644 spice-rack.md student@lab:~$ cd ~/team-recipes && printf '\nReplace ground spices every 12 months for best flavor.\n' >> spice-rack.md student@lab:~$ cd ~/team-recipes && git add spice-rack.md student@lab:~$ cd ~/team-recipes && git commit -m 'Add spice freshness guidance' [feature/spice-rack 6e71ad6] Add spice freshness guidance 1 file changed, 2 insertions(+) student@lab:~$ cd ~/team-recipes && git push -u origin feature/spice-rack Enumerating objects: 4, done. Counting objects: 100% (4/4), done. Compressing objects: 100% (3/3), done. Writing objects: 100% (3/3), 431 bytes | 431.00 KiB/s, done. Total 3 (delta 0), reused 0 (delta 0), pack-reused 0 To ../recipes-origin.git * [new branch] feature/spice-rack -> feature/spice-rack branch 'feature/spice-rack' set up to track 'origin/feature/spice-rack'.

🔀 Let Master Diverge Too

Add a genuinely independent commit directly to master.

cd ~/team-recipes && git switch master

cd ~/team-recipes && printf 'Freezer Labels\n\nDate and label everything before freezing.\n' > freezer-labels.md

cd ~/team-recipes && git add freezer-labels.md

cd ~/team-recipes && git commit -m 'Add freezer labeling recipe note'

cd ~/team-recipes && git push

student@lab:~$ cd ~/team-recipes && git switch master Switched to branch 'master' Your branch is up to date with 'origin/master'. student@lab:~$ cd ~/team-recipes && printf 'Freezer Labels\n\nDate and label everything before freezing.\n' > freezer-labels.md student@lab:~$ cd ~/team-recipes && git add freezer-labels.md student@lab:~$ cd ~/team-recipes && git commit -m 'Add freezer labeling recipe note' [master 80096d8] Add freezer labeling recipe note 1 file changed, 3 insertions(+) create mode 100644 freezer-labels.md student@lab:~$ cd ~/team-recipes && git push Enumerating objects: 4, done. Counting objects: 100% (4/4), done. Compressing objects: 100% (3/3), done. Writing objects: 100% (3/3), 342 bytes | 38.00 KiB/s, done. Total 3 (delta 1), reused 0 (delta 0), pack-reused 0 To ../recipes-origin.git 4a2fc3b..80096d8 master -> master

📍 Trace the Real Merge Base by Hand

Find the exact common ancestor, and confirm the three real commits git will actually compare.

cd ~/team-recipes && git merge-base master feature/spice-rack

cd ~/team-recipes && git log --oneline master -1

cd ~/team-recipes && git log --oneline feature/spice-rack -1

student@lab:~$ cd ~/team-recipes && git merge-base master feature/spice-rack 4a2fc3ba85ad051414aaf347eb7ce61cf943beab student@lab:~$ cd ~/team-recipes && git log --oneline master -1 80096d8 (HEAD -> master, origin/master, origin/HEAD) Add freezer labeling recipe note student@lab:~$ cd ~/team-recipes && git log --oneline feature/spice-rack -1 6e71ad6 (origin/feature/spice-rack, feature/spice-rack) Add spice freshness guidance

🔗 Perform the Real Three-Way Merge

Merge, and confirm git combined both genuinely independent changes with no conflict.

cd ~/team-recipes && git merge --no-ff feature/spice-rack -m 'Merge pull request: spice rack guide'

cd ~/team-recipes && git push

cd ~/team-recipes && git log --oneline --graph -5

student@lab:~$ cd ~/team-recipes && git merge --no-ff feature/spice-rack -m 'Merge pull request: spice rack guide' Merge made by the 'ort' strategy. spice-rack.md | 5 +++++ 1 file changed, 5 insertions(+) create mode 100644 spice-rack.md student@lab:~$ cd ~/team-recipes && git push Enumerating objects: 4, done. Counting objects: 100% (4/4), done. Compressing objects: 100% (2/2), done. Writing objects: 100% (2/2), 308 bytes | 102.00 KiB/s, done. Total 2 (delta 1), reused 0 (delta 0), pack-reused 0 To ../recipes-origin.git 80096d8..f18174f master -> master student@lab:~$ cd ~/team-recipes && git log --oneline --graph -5 * f18174f (HEAD -> master, origin/master, origin/HEAD) Merge pull request: spice rack guide |\ | * 6e71ad6 (origin/feature/spice-rack, feature/spice-rack) Add spice freshness guidance | * 6e16df9 Add spice rack organization guide * | 80096d8 Add freezer labeling recipe note |/ * 4a2fc3b Merge pull request: fridge organization tips |\