In the 90s of the last century high statistics experiments with fully equipped 4π detectors have lead to a better insight in the spectrum of hadrons. In particular the finding of crypto-exotic and J PC exotic states tremendously improved the experimental situation in meson spectroscopy. All this was possible only with sophisticated analysis methods like the decomposition of measured phase-space distribution into partial waves and to express the partial waves in terms of complicated dynamical

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... ctions. This paper gives an introduction about the concepts and formalisms involved. PACS 11.80 -Partial wave analysis.