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Degeneracy weight (or statistical weight, degeneracy weighting, or simply degeneracy) is a count of the quantum states of an atom (state of excitation) that have the same energy level. It is useful when the number of particles at a particular energy level is what's directly needed rather than their exact quantum states. The concept is used in determining how many otherwise-identical molecules occupy each possible energy level (e.g., within a gas), such as using the Boltzmann equation. The energy levels considered "the same" might be extremely close, but not exact, a difference that can lead to forbidden lines, fine structure and hyperfine structure.