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The 12 pages that link to Square matrix, each with the reason it gives.
DeterminantNarrower topic: Only square matrices have determinants in the standard definition.
Matrix inverseNarrower topic: Only square matrices have ordinary two-sided inverses.
Symmetric matrixNarrower topic: Only square matrices can equal their own transpose.
Triangular matrixNarrower topic: Triangularity is defined only for square matrices.
Leibniz formula for determinantsNarrower topic: The formula applies to square arrays of entries.