The first tile 1.1, and 1.2, 1.3, 1.4 obtained from it by 90°
rotations;
the mirror-image 1.5 of the first tile, and 1.6, 1.7, 1.8 obtained
from it by 90° rotations.
Tile 2.1 and its 90° rotation images 2.2, 2.3, 2.4 2.4.
The same principle is used for denoting other tiles, e.g.,
3.1 - 3.4.
4.1 - 4.8.
5.1 - 5.8.
6.1 - 6.8.
7.1 - 7.8.
8.1 - 8.4.
9.1 - 9.2.
10.1 - 10.2.