Domino

By cataloging all 27 dominoes visible in the image and comparing them to a full double-six set, the only missing combination is the [4, 6] domino.

The excluded domino is number 3 and 3

By systematically checking all 28 possible dominoes in a standard double-six set (all combinations of numbers 0-6), the domino [4, 4] is the only one not visible in the image. All other dominoes including [0, 0], [1, 1], [2, 2], [3, 3], [5, 5], [6, 6], and all mixed pairs are present in the 27 tiles shown.

After carefully examining all 27 dominoes in the image and comparing them to a complete standard double-six set (which contains 28 tiles), I systematically went through each possible combination from [0,0] to [6,6]. The blank-blank domino [0,0] does not appear in the image, making it the missing tile.

Counting pips on all visible tiles shows the number 4 appears two fewer times than it should in a full double-six set, indicating the double-4 (4,4) tile is missing.

A standard double-six domino set contains 28 unique tiles. By systematically identifying all 27 dominoes in the image and comparing against the complete set, every domino is accounted for except [1,6]. The image 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], [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], [6,6]. The missing domino is [1,6].