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We present a rigorous comparison of the unique characteristics of directional couplers and multimode interferometers based on the unique properties of high-index contrast ridge waveguides. The two devices are intimately related as the multimode interference (MMI) is derived from the directional couplers (DCs). We show for the first time the continuous evolution from the two-mode coupling characteristic of DC to the multimode mixing and interference characteristic of MMI, as the DC is structurally transformed into the MMI. We also show that DC can be designed to have the MMI features of compactness and polarization-insensitivity, two traits that reflect their shared lineage. The performance of MMI and DC are compared in terms of coupling length, polarization dependence, crosstalk, excess loss, and fabrication tolerances. We show that the DC, as long as it is designed to have nearly the same coupling length for transverse electric and transverse magnetic, can potentially have better performance than the MMI in terms of crosstalk and polarization sensitivity. Such a DC, however, requires careful control of many design parameters, while the MMI design is more robust and involves fewer design variables. Finally, the effect of higher-order modes and mode filtering are also considered.