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The Petrov type of a metric describes the algebraic properties of the Weyl tensor, the vacuum part of the Riemann tensor. The Petrov classification is based on the multiplicities of the null eigenvectors (principal null directions) of the Weyl tensor. A metric of Petrov type D is algebraically special, having two repeated (double) principal null vectors, while metrics of type I have no algebraic symmetry, possessing four simple principal null directions.