Because a Git commit can have more than 1 parent due to merge commits when you do:
git merge
It can even have more than 2, there's no limit. Although that is not so common (with good reason, 2 is already one too many): softwareengineering.stackexchange.com/questions/314215/can-a-git-commit-have-more-than-2-parents/377903#377903
There are two ways to organize a project:
Some people like merges, but they are ugly and stupid. Rebase instead and keep linear history.
Linear history:
5 master
|
4
|
3
|
2
|
1 first commit
Branched history:
7   master
|\
| \
6  \
|\  \
| |  |
3 4  5
| |  |
| /  /
|/  /
2  /
| /
1/  first commit
Here commits 6 and 7 are the so called "merge commits":
  • they have multiple parents:
    • 6 has parents 3 and 4
    • 7 has parents 5 and 6
  • they are useless and don't contain any real information
Which type of tree do you think will be easier to understand and maintain?
????
????????????
You may disconnect now if you still like branched history.
Generate a minimal test repo. You should get in the habit of doing this to test stuff out.
#!/usr/bin/env bash

mkdir git-tips
cd git-tips
git init

for i in 1 2 3 4 5; do
  echo $i > f
  git add f
  git commit -m $i
done

git checkout HEAD~2
git checkout -b my-feature

for i in 6 7; do
  echo $i > f
  git add f
  git commit -m $i
done
Git tips / gitk by Ciro Santilli 37 Updated 2025-07-16
For the newbs.
Slick? No. But gitk does the job, like any one of the other 100 billion free Git UI viewers out there
gitk master HEAD
https://raw.githubusercontent.com/cirosantilli/media/master/gitk.png
Many IDEs are also implementing this now (e.g. VS Code, Eclipse. Most free IDE GIt implementations are still crap, but that is the future, because you want to edit, view history, edit, view history, commit, edit.
For the strong.
git log --abbrev-commit --decorate --graph --pretty=oneline master HEAD
Output:
* b4ec057 (master) 5
* 0b37c1b 4
| * fbfbfe8 (HEAD -> my-feature) 7
| * 7b0f59d 6
|/
* 661cfab 3
* 6d748a9 2
* c5f8a2c 1
If we also add the --simplify-by-decoration, which you very often want want on a real repository with many commits:
* b4ec057 (master) 5
| * fbfbfe8 (HEAD -> my-feature) 7
|/
* c5f8a2c 1
As we can see, this removes any commit that is neither:
  • under a branch or tag
  • at the intersection of too branches or tags
Option 1) git commit. Doh!!!
Option 2) git rebase. Basically allows you to do arbitrary modifications to the tree. The most important ones are:
Before:
5 master
|
4 7 my-feature HEAD
| |
3 6
|/
2
|
1
Action:
git rebase
After:
7 my-feature HEAD
|
6
|
5 master
|
4
|
3
|
2
|
1
Ready to push with linear history!
Before:
7 my-feature HEAD
|
6
|
5 master
|
4
|
3
|
2
|
1
Oh, commit 6 was crap:
git rebase -i HEAD~2
Mark 6 to be modified.
After:
7 my-feature HEAD
|
6v2
|
5 master
|
4
|
3
|
2
|
1
Better now, ready to push.
Note: history changes change all commits SHAs. All parents are considereEven time is considered. So is commit message/author. And obviously file contents. So now commit "7" will actually have a different SHA.
Before
7 my-feature HEAD
|
6
|
5 master
|
4
|
3
|
2
|
1
Oh, commit 6 was just a temporary step, should be put together with commit 7:
git rebase -i HEAD~2
Mark 6 to be squashed.
After:
67 my-feature HEAD
|
5 master
|
4
|
3
|
2
|
1
Better now, ready to push.
Oh but there are usually 2 trees: local and remote.
So you also have to learn how to observe and modify and sync with the remote tree!
But basically:
git fetch
to update the remote tree. And then you can use it exactly like any other branch, except you prefix them with the remote (usually origin/*), e.g.:
  • origin/master is the latest fetch of the remote version of master
  • origin/my-feature is the latest fetch of the remote version of my-feature
The key to solve conflicts is:
You have to understand what are the two commits that touched a given line (one from master, one from features), and then combine them somehow.
Or in other words, at every rebase conflict we have something like:
master-commit    feature-commit
|                |
|                |
base-commit------+
|
|
Therefore there are 2 diffs that you have to understand and reconcile:
  • base-commit to master-commit
  • base-commit to feature-commit
Git tips / diff3 by Ciro Santilli 37 Updated 2025-07-16
diff3 conflict is basically what you always want to see, either by setting it as the default as per stackoverflow.com/questions/27417656/should-diff3-be-default-conflictstyle-on-git:
git config --global merge.conflictstyle diff3
or as a one off:
git checkout --conflict=diff3
With this, conflicts now show up as:
++<<<<<<< HEAD
 +5
++||||||| parent of 7b0f59d (6)
++3
++=======
+ 6
++>>>>>>> 7b0f59d (6)
7b0f59d is the SHA-2 of commit 6.
instead of the inferior default:
++<<<<<<< ours
 +5
++=======
+ 6
++>>>>>>> theirs
We can also observe the current tree state during resolution:
* b4ec057 (HEAD, master) 5
* 0b37c1b 4
| * fbfbfe8 (my-feature) 7
| * 7b0f59d 6
|/
* 661cfab 3
* 6d748a9 2
* c5f8a2c 1
so we understand that we are now at 5 and that we are trying to apply our commit 6
So it is much clearer what is happening:
  • master changed the code from 3 to 5
  • our feature changed the code from 3 to 6
and so now we have to decide what the new code is that will put both of these together.
Let's say we decide it is 5 + 6 = 11 and continue rebasing:
git add .
git rebase --continue
We now reach:
++<<<<<<< HEAD
 +11
++||||||| parent of fbfbfe8 (7)
++6
++=======
+ 7
++>>>>>>> fbfbfe8 (7)
and the tree looks like:
* ca7f7ff (HEAD) 6
* b4ec057 (master) 5
* 0b37c1b 4
| * fbfbfe8 (my-feature) 7
| * 7b0f59d 6
|/
* 661cfab 3
* 6d748a9 2
* c5f8a2c 1
So we understand that:
  • after the previous step we added commit 6 on top of 5
  • now we are adding 7 on top of the new 6 (which we decided would contain 11)
and after resolving that one we now reach:
* e1aaf20 (HEAD -> my-feature) 7
* ca7f7ff 6
* b4ec057 (master) 5
* 0b37c1b 4
* 661cfab 3
* 6d748a9 2
* c5f8a2c 1
These are good free newbie GUI options:
sudo apt install meld
git mergetool --tool meld

sudo apt install kdiff3
git mergetool --tool kdiff3
https://raw.githubusercontent.com/cirosantilli/media/master/meld.png
https://raw.githubusercontent.com/cirosantilli/media/master/kdiff3.png
Let's make a more interesting conflict:
git-tips-2.sh
#!/usr/bin/env bash

set -eux

add() (
  rm -f f
  for i in `seq 10`; do
    printf "before $i\n\n" >> f
  done
  printf "conflict 1 $1\n\n" >> f
  for i in `seq 10`; do
    printf "middle $i\n\n" >> f
  done
  printf "conflict 2 $2\n\n" >> f
  for i in `seq 10`; do
    printf "after $i\n\n" >> f
  done
  git add f
)

rm -rf git-tips-2
mkdir git-tips-2
cd git-tips-2
git init

for i in 1 2 3; do
  add $i $i
  git commit -m $i
done

add 3 4
git commit -m 4

add 5 4
git commit -m 5

git checkout HEAD~2
git checkout -b my-feature

add 3 6
git commit -m 6

add 7 6
git commit -m 7
git rebase does not tell you that, and that sucks.
We only know which commit from the feature branch caused the problem.
Generally we can guess or it is not needed, but imerge does look promising: stackoverflow.com/questions/18162930/how-can-i-find-out-which-git-commits-cause-conflicts
Web technology by Ciro Santilli 37 Updated 2025-07-16
Old cheat on separate repo: web.
Now moving to either:
  • separate files under: web-cheat/ for the boring stuff
  • subsections under this section for the more exciting stuff!

Pinned article: Introduction to the OurBigBook Project

Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.
Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!
We have two killer features:
  1. topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculus
    Articles of different users are sorted by upvote within each article page. This feature is a bit like:
    • a Wikipedia where each user can have their own version of each article
    • a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
    This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
    Figure 1.
    Screenshot of the "Derivative" topic page
    . View it live at: ourbigbook.com/go/topic/derivative
  2. local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:
    This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
    Figure 2.
    You can publish local OurBigBook lightweight markup files to either https://OurBigBook.com or as a static website
    .
    Figure 3.
    Visual Studio Code extension installation
    .
    Figure 4.
    Visual Studio Code extension tree navigation
    .
    Figure 5.
    Web editor
    . You can also edit articles on the Web editor without installing anything locally.
    Video 3.
    Edit locally and publish demo
    . Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension.
    Video 4.
    OurBigBook Visual Studio Code extension editing and navigation demo
    . Source.
  3. https://raw.githubusercontent.com/ourbigbook/ourbigbook-media/master/feature/x/hilbert-space-arrow.png
  4. Infinitely deep tables of contents:
    Figure 6.
    Dynamic article tree with infinitely deep table of contents
    .
    Descendant pages can also show up as toplevel e.g.: ourbigbook.com/cirosantilli/chordate-subclade
All our software is open source and hosted at: github.com/ourbigbook/ourbigbook
Further documentation can be found at: docs.ourbigbook.com
Feel free to reach our to us for any help or suggestions: docs.ourbigbook.com/#contact