Landau's classification of phases uses expectation values of local operators to determine the phase of the system. However, Elitzur's theorem shows that this approach is inadmissible in certain systems such as Yang–Mills theories for which no local operator can act as an order operator for confinement. Instead, to get around the theorem requires constructing nonlocal gauge invariant operators, whose expectation values need not be zero. The most common ones are Wilson loops and their thermal equivalents, Polyakov loops. Another nonlocal operator that acts as a order operator is the 't Hooft loop.