Branches - Git Recipes - page 196

How It Works
Every two branches in a repository can be characterized as either merged or not
merged . We say that branch a is merged into branch b if all the revisions included in a
are also included in b . Figure 5-15 presents two branches a and b . Branch a is merged
in branch b . Branch a contains two revisions a1 and a2 —both of them can be found
starting from the revision pointed to by b and following parent revisions.
Figure 5-15 . Branch a is merged into branch b; branch b is not merged into branch a
This relation usually is not reflective. In the repository in Figure 5-15 branch b is
not merged in the master . As you can see, the revisions b1 , b2 , and b3 are included
in branch b , but not in branch a . The only case when this relation is reflective is when
both branches point to the exactly the same revision, as in Figure 5-16 .