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Three correlations and a samosa: https://picturethismaths.wordpress.com/2020/10/31/three-correlations-and-a-samosa/

3x3 symmetric matrices with unit diagonals form a three-dimensional linear space, in which the samosa is a curvy 3d convex set representing the positive definite matrices. Taking sections of it allows you to infer the possible correlations between two variables, given each of their correlations with a third. See also https://picturethismaths.wordpress.com/2021/02/03/three-correlations-and-other-statistical-models/ and https://picturethismaths.wordpress.com/2021/03/18/three-correlations-and-an-algebraic-classification/