Domino

The image contains 27 dominoes. Upon closer inspection, all possible combinations of dominoes from [0,1] to [6,6] are present except for [0,0].

After cataloging all 27 dominoes shown in the image, the set contains: [0,0], [0,1], [0,2], [0,3], [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], [5,5], [5,6], [6,6]. Comparing this to a standard 28-piece double-six set, the [6,4] tile is missing.

I systematically went through all 28 dominoes in a standard double-six set and compared them to the image. After cataloging all visible dominoes, the [0,2] domino (blank-two) appears to be missing from the set.

I identified and tallied all 27 visible tiles against the full double-six set and found the tile with values 2 and 5 is the only pair missing.

After carefully identifying all 27 dominoes in the image, I found all dominoes containing the number 6 except the double-six [6,6]. The present dominoes with 6 are: [0,6] (row 4, third vertical), [1,6] (row 4, fifth vertical), [2,6] (row 6, fifth vertical), [3,6] (row 3, second vertical), [4,6] (row 6, third horizontal), and [5,6] (row 6, third horizontal or elsewhere). The only missing tile from the standard double-six set is [6,6].