Domino

The excluded domino is number 0 and 0

After cataloging all 27 dominoes shown (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, 6-6, 6-5), the only tile from the standard 28-piece double-six set missing is the double-five (5-5).

By systematically identifying all 27 visible dominoes in the image and comparing them against the complete set of 28 unique dominoes in a standard double-six set, the domino [2, 4] is the only tile missing from the set. All other dominoes from [0,0] through [6,6] are present in the image.

I systematically identified all 27 dominoes visible in the image and compared them against the full set of 28 dominoes. Going through all pairs from [0,0] to [6,6], I found that the double-blank [0,0] domino is missing from the image.

I counted all visible tiles and identified every pair from 0–6 except the tile pairing 4 and 6, so the missing domino is [4,6]

By systematically identifying all dominoes in the image and comparing against the complete set of 28 double-six dominoes, I found that all dominoes are present except [0,6]. Key identifications include: [6,6], [5,5], [4,4], [3,3], [2,2], [1,1], [0,0] (all doubles present); [1,6], [3,6] in the bottom row; [0,4] as the vertical domino at bottom left; [2,4] in the bottom row; [4,5], [3,5], [2,5], [1,5], [0,5]; [3,4], [2,3], [1,4], [1,3], [1,2], [0,3], [0,2], [0,1]. The only domino from the complete set not found in the image is [0,6].