Single quote usage for example Git commit refs in documentation

Question

For example, this question, has the following diagram

M1  - M2 - M3
   \         \
    \         \
     A1 - A2   A1' - A2'

What does the single quote on A1 and A2, i.e. A1' and A2', in this context mean?

Answer 1

It's a symbol borrowed from the mathematical notation of a derivative.

A1', in this example, is the result of rebasing A1 on top of M3. While they will probably have the same body and same commit message (assuming there were no merge conflicts during the rebase), they have different parents (A1's parent is M1 and A1''s parent is M3), and thus they are not the same commit, and do not have the same commit hash. A1' is derived from the original A1, and this common mathematical notation is used to highlight it.