How to read git log graph

In the git community book, it says

Another interesting thing you can do is visualize the commit graph with the '--graph' option, like so:

$ git log --pretty=format:'%h : %s' --graph
* 2d3acf9 : ignore errors from SIGCHLD on trap
*   5e3ee11 : Merge branch 'master' of git://github.com/dustin/grit
|\
| * 420eac9 : Added a method for getting the current branch.
* | 30e367c : timeout code and tests
* | 5a09431 : add timeout protection to grit
* | e1193f8 : support for heads with slashes in them
|/
* d6016bc : require time for xmlschema

It will give a pretty nice ASCII representation of the commit history lines.

How should I read this graph? How does 420eac9 differ from the rest?

Solution 1:

The asterisks show where something was committed:

e1193f8, 5a09431 and 30e367c were committed to the left branch (yielding a | on the right branch) whereas 420eac9 was committed to the right branch (yielding a | on the left branch). And that is how 420eac9 differs from the rest: it's the only commit to the right branch.

For the sake of completeness:

• d6016bc was the branching point
• 5e3ee11 is the merging commit
• 2d3acf9 is the first commit after merging

Solution 2:

420eac9 is on a different branch than the 3 commits "below" it. The branches diverged after d6016bc and they were merged in 5e3ee11.