Domino

After cataloging all 27 dominoes 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]. The only missing unique tile from the standard 28-tile double-six set is the [4,6] (or [6,4]) domino.

After systematically cataloging all 27 visible dominoes and comparing them to the full standard set of 28 tiles, the domino [0,5] (blank-five) is the one that does not appear in the image.

The excluded domino is number 0 and 0

By systematically examining all 27 dominoes visible in the image and cross-referencing against the complete standard set of 28 unique tiles (representing all pairs of numbers 0-6), the double-four domino [4, 4] is the only tile missing from the set.

After examining the pictured set of 27 tiles, the only domino missing from the complete double-six set is the double six (6,6).

The excluded domino is number 5 and 3

After systematically identifying all 27 visible dominoes in the image, I accounted for all dominoes in the standard double-six set except [6,6]. The complete set includes: [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], [4,6], [5,5], [5,6]. The only domino from the full set of 28 that is not present is [6,6].