Domino

After cataloging all 27 dominoes in the image and comparing them against a standard double-six set, all tiles featuring blank (0), 1, 2, 3, and 5 are present in their complete combinations. The double blanks [0,0], double ones [1,1], through double sixes [6,6] are accounted for. When checking the combinations for 4 and 6, the domino [4,6] (or [6,4]) is missing from the collection.

The excluded domino is number 2 and 1

By systematically checking all 28 possible domino combinations in a standard double-six set, the domino [4, 4] is the only one missing from the image. All other dominoes from [0, 0] through [6, 6] can be identified among the 27 tiles shown.

I systematically catalogued all visible dominoes in the image and compared them against the complete set of 28 dominoes. After identifying all tiles present, the domino [0,3] (blank-three) was the only one not found in the image.

After counting the visible ends for each pip value, the sixes appear one time fewer than they should (7 instead of 8), so the missing tile is the double-six [6,6].

After systematically identifying all 27 visible dominoes in the image, I found matches for every tile in a standard double-six set except [6,6]. The dominoes [0,0], [0,1], [0,2], [0,3], [0,4], [0,5], [0,6] are all present in various rows. The doubles [1,1], [2,2], [3,3], [4,4], and [5,5] are present. All combinations with 6 are present ([0,6], [1,6], [2,6], [3,6], [4,6], [5,6]) except the double [6,6]. The domino [6,6] would display six dots on both halves, and no such tile appears in the image.