Why a Double-Six Set Has 28 Tiles
Each end of a domino can show any number from zero through six. A tile is an unordered pair: the 2–5 tile is the same tile as 5–2, because turning it around does not create a new pairing. Doubles count too, with both ends showing the same number.
That makes the set a combinations problem. There are seven possible values, and we need to choose two with repetition allowed. The count is 7 × 8 ÷ 2, or 28: the division removes the duplicate orientations, while the extra factor allows pairs such as 4–4.
The Pattern Extends to Larger Sets
The same rule explains other standard domino families. If the highest number is n, there are n + 1 possible values, so the set contains (n + 1)(n + 2) ÷ 2 distinct tiles. A double-nine set, for example, adds values through nine and contains 55 tiles.
The inventory is therefore not an arbitrary handful of pieces: it is the complete list of possible pairings within the set’s number range. That completeness is what lets players draw and play tiles without encountering a pairing the set was supposed to contain.
