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, and a 1x2 rectangle. Just Look at it! 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°.
In a wider sense, the term domino is sometimes understood to mean a tile of any shape.
Packing and tiling
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 nth Fibonacci number.Concrete Mathematics by Graham, Knuth and Patashnik, Addison-Wesley, 1994, p. 320,
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, 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..
See also Dominoes, a set of domino-shaped gaming pieces Tatami, Japanese domino-shaped floor mats
References