Domino

By systematically checking all 28 possible domino combinations in a standard double-six set (0-0 through 6-6), I identified all dominoes present in the image. The domino [2, 3] (or equivalently [3, 2]) is the only combination that does not appear anywhere in the 27 tiles shown.

By systematically cataloging all visible dominoes in the image and comparing against the full 28-tile standard double-six set, the tile [1,4] (or equivalently [4,1]) is the one not present among the 27 shown dominoes.

The image contains 27 dominoes. Upon examining the image, it is observed that all possible combinations of numbers from 0 to 6 are represented except for the double-zero domino, which is the domino with 0 on both halves.

After listing all 28 tiles in a standard double-six set and comparing them to the 27 tiles present in the image, the [6, 6] (double-six) tile is the only one missing.

All other double tiles (0-0 through 5-5) and the non-double combinations are present in the image but the double-six (6,6) is not visible, so the missing domino is 6 and 6.

Using the ASCII representation where each dot is counted (``:`` = 2 dots, ``::`` = 4 dots, ``:::`` = 6 dots, ``:.`` = 3 dots, ``::.`` = 5 dots, ``.`` = 1 dot), I identified all 27 dominoes in the image. The complete double-six set contains 28 dominoes. After cataloging the present dominoes including [0,0], [0,2], [0,5], [0,6], and [1,2] through [1,6], the domino [0,1] (blank on one side, single dot on the other) is the only one missing from the standard set.