We introduce the general two-Higgs-doublet model effective field theory where the effects of new physics are parametrized by non-renormalizable operators up to mass dimension-six. Within this general framework, we calculate the physical Higgs spectrum, contributions to the couplings and masses of electroweak gauge bosons and fermions, and a complete set of Feynman rules. We discuss differences of the effective field in four types of two-Higgs-doublet models distinguished by Z2 symmetries which restrict the couplings of the Higgs doublets to Standard Model fermions: type-I; -II; -X; and -Y. We demonstrate the advantages of transforming the effective theory into the Higgs basis, including the separation of operators that modify Standard Model couplings and masses from operators that only contribute to scattering processes, transparent relations between scattering processes resulting from the same operator, and the explanation of correlations between different operators in specific UV completions.
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