The definition of the cotetrad uses the usual abuse of notation d x μ μ = d φ φ μ μ to define covectors (1-forms) on M . The involvement of the coordinate tetrad is not usually made explicit in the standard formalism. In the tetrad formalism, instead of writing tensor equations out fully (including tetrad elements and tensor products ⊗ ⊗ as above) only components of the tensors are mentioned. For example, the metric is written as " g a b ". When the tetrad is unspecified this becomes a matter of specifying the type of the tensor called abstract index notation. It allows to easily specify contraction between tensors by repeating indices as in the Einstein summation convention.