In a quantum system in a pure state, a subsystem generally has a nonzero entropy because of entanglement with the rest of the system. Is the average entanglement entropy of pure states also the typical entropy of the subsystem? We present a method to compute the exact formula of the momenta of the probability P (SA)dSA that a subsystem has entanglement entropy SA. The method applies to subsystems defined by a subalgebra of observables with a center. In the case of a trivial center, we reobtain the well-known result for the average entropy and the formula for the variance.In the presence of a nontrivial center, the Hilbert space does not have a tensor product structure and the well-known formula does not apply. We present the exact formula for the average entanglement entropy and its variance in the presence of a center. We show that for large systems the variance is small, ∆SA/ SA 1, and therefore the average entanglement entropy is typical. We compare exact and numerical results for the probability distribution and comment on the relation to previous results on concentration of measure bounds. We discuss the application to physical systems where a center arises. In particular, for a system of noninteracting spins in a magnetic field and for a free quantum field, we show how the thermal entropy arises as the typical entanglement entropy of energy eigenstates.Introduction.-In a seminal paper [1], Page showed that, when an isolated quantum system is in a random pure state, the average entropy of a subsystem is close to maximal. This result plays a central role in the analysis of the black hole information puzzle [2][3][4][5][6][7][8][9], in the quantum foundations of statistical mechanics [10][11][12][13][14][15][16][17][18][19][20][21][22][23], in quantum information theory [24][25][26][27][28][29][30][31] and in the study of the quantum nature of spacetime geometry [32][33][34][35][36][37]. The entanglement entropy, expressed as a function of the subsystem size, is often called the Page curve ( Fig. 1). In a quantum system in a pure state, a subsystem A generally has a nonzero entropy because of entanglement with the rest of the system. In this paper we address the question: Is the average entanglement entropy of pure states S A also the typical entropy of the subsystem? To illustrate the significance of this question, let us consider for instance the gas in a room held at fixed temperature. The canonical ensemble allows us to compute the average energy of the gas. However, the configuration of molecules in one room is one realization of this ensemble-we are not averaging over rooms. How close to the average is the energy of this realization? In other words, is the average energy typical? In statistical mechanics, we answer this question by computing the variance (∆E) 2 . In the canonical ensemble, we find ∆E/ E 1 and we conclude that the average energy is typical. Here we investigate the typicality of the entanglement entropy S A of a subsystem by studying its average and variance.A well-studied special case of ...