Whats the Diff?

 
With the index there are two different places to diff
 
 (1) $ git diff 
 (2) $ git diff HEAD
 (3) $ git diff HEAD^ HEAD
 (4) $ git diff HEAD^^ HEAD^
 
 (1) diff between cwd and index
 (2) diff between cwd and the HEAD
 (3) diff between previous HEAD and HEAD
 (4) ???