+ - - - + - - - + - - - +
| x x x | x x x | x x x |
| x x x | x x x | x x x |
| x x x | x x x | x x x |
+ - - - + - - - + - - - +
| x x x | x x x | x x x |
| x x x | x x x | x x x |
| x x x | x x x | x x x |
+ - - - + - - - + - - - +
| x x x | x x x | x x x |
| x x x | x x x | x x x |
| x x x | x x x | x x x |
+ - - - + - - - + - - - +