There are several different clubs/associations where the American Bulldog can be registred. These clubs/associations have usally their own written standard, slightly differing from the next.
For the different American Bulldog standards I suggest that you visit the standards page of American Bulldog Outpost where the standards of the following registries can be found:
-American Bulldog National Alliance
-American Bulldog Association Breed Standard
-National American Bulldog Association Breed Standard
-National Kennel Club Breed Standard
-Animal Research Foundation Breed Standard