+There are two ways to specify which commits to operate on.
+
+1. A single commit, <since>, specifies that the commits leading
+ to the tip of the current branch that are not in the history
+ that leads to the <since> to be output.
+
+2. Generic <revision range> expression (see "SPECIFYING
+ REVISIONS" section in gitlink:git-rev-parse[1]) means the
+ commits in the specified range. A single commit, when
+ interpreted as a <revision range> expression, means
+ "everything that leads to that commit", but that is taken as
+ the special case above. If you want to format everything
+ since project inception to one commit, say "git format-patch
+ \--root <that-commit>", as showing the root commit as patch
+ requires \--root option anyway.
+