Domino

The excluded domino is number 2 and 2

A standard double-six domino set consists of 28 unique tiles including all combinations from [0,0] to [6,6]. By cataloging all 27 tiles visible in the image, it is determined that the [4,4] (double-four) tile is missing from the set.

I systematically identified all 28 dominoes in a standard double-six set and compared them to the tiles visible in the image. After cataloging each domino present, the tile [0,3] (blank-3) was the only one not found among the 27 shown.

By systematically checking all 28 possible combinations of dominoes in a standard double-six set (0-0 through 6-6), I identified 27 dominoes present in the image. The domino [2,4] appears to be missing from the complete set. All other dominoes including [0,0], [1,1], [2,2], [3,3], [4,4], [5,5], [6,6], [0,1], [0,2], [0,3], [0,4], [0,5], [0,6], [1,2], [1,3], [1,4], [1,5], [1,6], [2,3], [2,5], [2,6], [3,4], [3,5], [3,6], [4,5], [4,6], and [5,6] are present.

After identifying and listing each domino shown and comparing to the full double-six set, the only missing tile is the one with 2 and 5.

Analyzing the grid of dominoes, I identified that most dominoes appear only once in the pattern, but several appear as duplicates: [2,3], [3,6], [2,2], [1,5], and [4,4]. Specifically looking at the doubles (where both halves are equal), the set includes [0,0], [1,1], [2,2], [3,3], [4,4], [5,5], and [6,6]. However, [2,2] appears twice (Row 1 and Row 5, both in Column 4) and [4,4] appears twice (Row 4 Column 3 and Row 6 Column 5). The duplicate [4,4] in Row 6 Column 5 is positioned at the end of the last row and breaks the pattern where all other domino types should be unique in the set. Additionally, the duplicate [2,2] maintains vertical alignment with its counterpart in Column 4, while the duplicate [4,4] does not align with its counterpart (Row 4 C3 vs Row 6 C5), making it the clearly excluded element that doesn’t fit the intended pattern of the puzzle.