A new formulation of general model reduction in structural dynamics is presented. The proposed method consists of four attributes: a hybrid method combining dynamic condensation and component mode synthesis that are arguably the most popular model reduction techniques, a precise model reduction taking the inertia effect into consideration, primal assembly at the boundary interface of substructures and general formulations of model reduction with primal assembly. Comparisons of the proposed model reduction method with the conventional methods are provided as well as an efficient model reduction procedure of the proposed method. In particular, its advantages and performance of the proposed method are thoroughly investigated with various numerical examples.