4.1 Revision numbers
Each version of a file has a unique revision
number. Revision numbers look like `1.1',
`1.2', `1.3.2.2' or even `1.3.2.2.4.5'.
A revision number always has an even number of
periodseparated decimal integers. By default revision
1.1 is the first revision of a file. Each successive
revision is given a new number by increasing the
rightmost number by one. The following figure displays
a few revisions, with newer revisions to the right.
 ++ ++ ++ ++ ++
! 1.1 !! 1.2 !! 1.3 !! 1.4 !! 1.5 !
++ ++ ++ ++ ++

It is also possible to end up with numbers containing
more than one period, for example `1.3.2.2'. Such
revisions represent revisions on branches
(see section Branching and merging); such revision numbers
are explained in detail in Branches and revisions.