[Formula: see text] statistical ensemble: systems with fluctuating energy, particle number, and volume.

Within the theory of statistical ensemble, the so-called [Formula: see text] ensemble describes equilibrium systems that exchange energy, particles, and volume with the surrounding. General, model-independent features of volume and particle number statistics are derived. Non-analytic points of the partition function are discussed in connection with divergent fluctuations and ensemble equivalence. Quantum and classical ideal gases, and a model of Bose gas with mean-field interactions are discussed as examples of the above considerations.