Git tool `tig`: What do those letters in the graph mean?

Question

When I run tig, I see a graph of the branches. It uses letters on those lines to show what happened there:

• “o” means commit
• “M“ is a merge
• “I” is a merge

What is the difference between “M” and “I”?

Answer 1

The I is labelled as initial in tig source code

It seems it marks the initial commit (common ancestor) of a merge. The M marks the merging commit.