This function is equal to unity when x = x ′ and decreases as the distance | x − − x ′ | increases. Typically, it decays exponentially to zero at large distances, and the system is considered to be disordered. But if the correlation function decays to a constant value at large | x − − x ′ | then the system is said to possess long-range order. If it decays to zero as a power of the distance then it is called quasi-long-range order (for details see Chapter 11 in the textbook cited below. See also Berezinskii–Kosterlitz–Thouless transition). Note that what constitutes a large value of | x − − x ′ | is understood in the sense of asymptotics.