Domino

The excluded domino is number 2 and 1

A standard double-six domino set contains 28 tiles. By cataloging the tiles in the image, we find: [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], and [5,6]. The only tile missing from the set is the double-six tile [6,6].

By systematically checking all 28 possible dominoes in a standard double-six set (all pairs from 0-6), the domino [0, 1] (or [1, 0]) is the only one not present in the image. All other 27 dominoes can be accounted for: the six doubles [0,0] through [6,6], and all 21 unique non-double combinations.

I inspected the 27 visible tiles and matched them against the complete double-six set; the only tile not present is the double four (4-4).

A standard double-six domino set contains 28 unique tiles. I systematically analyzed the image to identify all present dominoes. The complete set should include all pairs from [0,0] to [6,6]. I found dominoes with values: [1,4], [1,3], [0,1], [2,2], [4,3], [0,3], [5,3], [0,5], [2,4], [0,0], [0,2], [3,3], [0,4], [1,2], [0,6], [5,5], [1,1], [1,5], [1,6], [2,5], [3,6], [5,6], [6,6], [3,4], [0,2], [4,5], [1,2], [2,3], [2,6]. After checking all combinations, the only missing domino is [4,4] (double-four), which should have 4 dots on both halves.