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In mathematics, a **domino** is a polyomino of order 2, that is, a polygon in the plane made of two equal-sized squares connected edge-to-edge.^{[1]} When rotations and reflections are not considered to be distinct shapes, there is only one *free* domino.

Since it has reflection symmetry, it is also the only *one-sided* domino (with reflections considered distinct). When rotations are also considered distinct, there are two *fixed* dominoes: The second one can be created by rotating the one above by 90°.^{[2]}^{[3]}

In a wider sense, the term *domino* is sometimes understood to mean a tile of any shape.^{[4]}

Dominos can tile the plane in a countably infinite number of ways. The number of tilings of a 2×*n* rectangle with dominoes is , the *n*th Fibonacci number.^{[5]}

Domino tilings figure in several celebrated problems, including the Aztec diamond problem in which large diamond-shaped regions have a number of tilings equal to a power of two,^{[6]} with most tilings appearing random within a central circular region and having a more regular structure outside of this "arctic circle", and the mutilated chessboard problem, in which removing two opposite corners from a chessboard makes it impossible to tile with dominoes.^{[7]}

**^**Golomb, Solomon W. (1994).*Polyominoes*(2nd ed.). Princeton, New Jersey: Princeton University Press. ISBN 0-691-02444-8.**^**Weisstein, Eric W. "Domino". From MathWorld – A Wolfram Web Resource. Retrieved 2009-12-05.**^**Redelmeier, D. Hugh (1981). "Counting polyominoes: yet another attack".*Discrete Mathematics*.**36**(2): 191–203. doi:10.1016/0012-365X(81)90237-5.**^**Berger, Robert (1966). "The undecidability of the Domino Problem".*Memoirs Am. Math. Soc*.**66**.**^***Concrete Mathematics Archived 2020-11-06 at the Wayback Machine*by Graham, Knuth and Patashnik, Addison-Wesley, 1994, p. 320, ISBN 0-201-55802-5**^**Elkies, Noam; Kuperberg, Greg; Larsen, Michael; Propp, James (1992), "Alternating-sign matrices and domino tilings. I",*Journal of Algebraic Combinatorics*,**1**(2): 111–132, doi:10.1023/A:1022420103267, MR 1226347**^**Mendelsohn, N. S. (2004), "Tiling with dominoes",*The College Mathematics Journal*,**35**(2), Mathematical Association of America: 115–120, doi:10.2307/4146865, JSTOR 4146865.