  The formalism of the exact six polarization modes of gravitational waves is
constructed in terms of both the small metric perturbations and the
Newman-Penrose scalars. The obtained formulae are applicable to any
metric-compatible gravity theories whose gravitational waves propagate along
either the null or non-null geodesics. Once a gravity theory, specifically its
linearized wave equation, is written, comparison to the observed data of the
laser interferometer experiments is direct.
