Domino

After listing all 27 tiles present in the image and comparing them against a full double-six set, the tile with numbers 6 and 1 is the only one missing.

The excluded domino is number 2 and 5

After systematically cataloging all 27 visible dominoes in the image, I identified that the domino [2, 4] (or equivalently [4, 2]) is missing from the standard double-six set of 28 dominoes. The set contains all other combinations of numbers 0-6, including all doubles [0-0] through [6-6] and all other unique pairs, but the [2, 4] domino is absent.

I systematically identified all 28 dominoes in a standard double-six set and compared them to the dominoes visible in the image. After cataloging all visible dominoes, the [0,3] domino (blank-three) appears to be missing from the set.

I inspected all 27 tiles and found representations for every unordered pair (0–6) except the tile pairing 2 with 5.

A standard double-six domino set contains 28 unique tiles. By examining the 27 dominoes shown in the image, I identified all the double tiles: [0,0] (vertical blank domino in row 4), [1,1] (vertical domino with one dot on each half in row 3), [3,3] (horizontal domino with three dots on each half in row 4), [4,4] (vertical domino with four dots on each half in row 4), [5,5] (horizontal domino with five dots on each half in row 5), and [6,6] (horizontal domino in the top right). The only double missing from this complete set is [2,2]. All other non-double dominoes are also accounted for in the image, confirming that [2,2] is the excluded domino.