Domino

By cataloging all 27 dominoes shown in the image and comparing them to a complete double-six set, which consists of 28 tiles, the [4, 4] (double-four) tile is the only one not present.

By systematically checking all 28 possible dominoes in a standard double-six set (all unique pairs from 0-6), I identified all 27 dominoes visible in the image. The domino [1, 1] (double-one) is the only domino from the complete set that is not present in the image.

I counted each visible tile and compared to a complete double-six set; every domino pair from 0–6 appears except the double six (6,6), so the excluded tile is [6,6].

By counting the occurrences of each number 0-6 on all domino halves in the image: The number 0 appears 7 times instead of the expected 8 (found in [0,0], [0,1], [0,2], [0,4], [0,5], and one other [0,x]), indicating a domino with 0 is missing. The number 3 also appears 7 times instead of 8 (found in [1,3], [2,3], [3,3], [3,4], [3,5], [3,6], and other [3,x] combinations), indicating the missing domino also contains 3. Since the missing domino must be the one containing both 0 and 3, the excluded domino is [0,3].