Domino

The excluded domino is number 2 and 1

By systematically checking against a standard double-six set, all double combinations (0-0, 1-1, 2-2, 3-3, 4-4, 5-5) are present except for the double-six tile.

By systematically checking all 28 possible dominoes in a standard double-six set (all pairs from 0-6), the double-four (4|4) is the only domino that is not visible in the image. All other dominoes, including 0-0, 1-1, 2-2, 3-3, 5-5, 6-6, and all combinations of different numbers, are present in the collection shown.

I systematically catalogued all visible dominoes in the image. A standard double-six set has 28 tiles. After identifying all 27 visible dominoes, the missing one is [0,3] (blank-3).

Counting pips on the shown tiles, the numbers 2 and 5 each appear only six times while every other number appears seven times, so the missing domino is 2-5.

The image contains 27 of the 28 standard dominoes. By examining the 0-series dominoes, I can identify [0,0], [0,1], [0,2], [0,3], [0,4], and [0,6] present in the image, but [0,5] is missing. Specifically: [0,0] appears in row 3 (right side), [0,1] in row 2 (right side), [0,2] in row 1 (third position), [0,3] in row 1 (first position), [0,4] in row 6 (fourth position), and [0,6] in row 3 (fourth position). The domino [0,5] is the only one from the complete set that is not present.