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Motivation for this Letter comes from two experiments. The first, by Kim and Chan (Phys. Rev. Lett. 53, 170 (1984)), measured a two-dimensional (2D) liquid-vapour critical point exponent. The second studied, via the magnetism of ultrathin metal films, the crossover of the critical exponent beta from 2D to 3D. Here, the analogy between magnetic behaviour near criticality and the corresponding liquid-vapour behaviour is first used to discuss the 2D-3D crossover in the latter case. Finally, the experimentally observed magnetic behaviour near criticality is considered for the ferromagnet CrBr(3) to allow fingerprints of the 3D Ising Hamiltonian to be anticipated.