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Traditionally, digital modulation schemes are bijective (i.e., the signal constellation points are disjunct), and the mapping is unique. Only recently it has been discovered that non-bijective modulation schemes may outperform bijective modulation schemes when employed in conjunction with suitable channel coding and iterative processing. In this in-depth tutorial, we clarify myths and facts about non-bijective modulation. Emphasis is on superposition modulation (SM). Without active signal shaping, SM outperforms bit-interleaved coded modulation with PSK or square QAM modulation at even lower receiver complexity.