There's a myth about Max Martin's melodic math that all phrase repetitions must be precise. That's not true--while repetitions are very close, Max 'tolerates' two kinds of melodic variations on the core motif: 1. Variations that allow lyrical flexibility WITHOUT harming the integrity of the core motif. 2. Variations that change the melodic function of the phrase and allow it to fit into a melodic structure (e.g., ABAC). Those have more freedom to tweak core notes but it still has to sound like a repetition. Here's an example from Taylor Swift's ​"Shake It Off" (2014).



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