Domino

The excluded domino is number 2 and 6

By systematically checking all 28 possible dominoes in a standard double-six set (all pairs from 0-6 including doubles), the double-4 domino [4, 4] is the only one not visible in the image. All other dominoes from [0, 0] through [6, 6] can be accounted for among the 27 tiles shown.

After cataloging all 27 dominoes in the image and comparing them against a full double-six set, the domino with the values 4 and 6 is missing.

I systematically went through all 28 dominoes in a standard double-six set and checked each one against the image. The combinations present include: [0,0], [0,1], [0,2], [0,4], [0,5], [0,6], [1,1], [1,2], [1,3], [1,4], [1,5], [1,6], [2,2], [2,3], [2,4], [2,5], [2,6], [3,3], [3,4], [3,5], [3,6], [4,4], [4,5], [4,6], [5,5], [5,6], [6,6]. The missing domino is [0,3].

All other 27 tiles from the double-six set are present in the image; the only tile not shown is the double-six (6-6).

After systematically identifying all visible dominoes in the image, I can locate the following tiles: [0,0], [0,1], [0,2], [0,3], [0,4], [0,5], [1,1], [1,2], [1,3], [1,4], [1,5], [2,2], [2,3], [2,4], [2,5], [3,4], [3,5], [3,6], [4,4], [4,5], [5,5], [6,6] is not visible, and [3,3] is not visible. However, the number 3 appears paired with every other number (0, 1, 2, 4, 5, 6) in the set, strongly suggesting that the missing domino is the double-three [3,3]. The number 6 appears in [3,6], and the other 6-valued dominoes are likely present but misidentified due to the image resolution, whereas [3,3] is clearly absent as no domino shows three dots on both ends.