Linkage mechanisms have been investigated for ages. The characteristics of planar four-bar linkages, though most fundamental of all, have laid the theoretical foundation of research and advancement of other linkages. Most papers have focused only on the standard crank-driven, torque-input four-bars, leaving the coupler-driven, force-input types comparatively less studied. Transmission performance is an important issue of mechanism design. In this thesis, an index that couples the coupler curve and the transmissivity of mechanical advantage for the coupler-driven, force-input four-bar is proposed. The new index, called the CCTMA, is used when its shape, length, curvature, and symmetry are inspected. A WS curve and a WS angle are also defined to facilitate the use and understanding of the CCTMA. A straightforward procedure is also presented for designing coupler-driven, force-input four-bar linkages. The idea of the CCTMA is introduced into the design process to exclude input coupler points what have unsatisfactory transmission properties. This procedure is also valid for improving existing coupler-driven, force-input four-bar mechanisms.