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This paper presents analytical descriptions of the behavior of weakly-guiding/weakly-coupled mode-selective fiber couplers, based on coupled-mode theory. Mode-selective couplers rely on the phase-matching of a higher-order mode in one fiber with the fundamental mode of a second, closely-positioned fiber. Their behavior is shown to be highly-dependent on the principle and azimuthal mode numbers of the higher-order mode, as well as its spatial-orientation. Zero coupling is shown to be possible for an asymmetric higher-order mode even when there is perfect phase-matching. The theory is also numerically simulated, and could assist in the future design of efficient mode-selective couplers for a wide range of optical communications and sensor systems.