The growth rate of double-diffusive interleaving has been evaluated using Stern's (1967) model which assumes vertical T/S gradients in the salt-fingering domain and compensating horizontal T/S gradients. The interleaving may take place along planes tilted with respect to the horizontal. For a given vertical scale, various cross-gradient and along-gradient tilts results in a variety of modes including two different growing modes, a decaying mode, and growing or damped vertically propagating modes. The relative importance of the process causing momentum, salinity, and temperature changes is discussed for each mode. A quartic equation is obtained for the tilt at which the growth rate is maximum. Near-maximum growth rates occur at widely differing along-gradient tilts of either sign, but the cross-gradient tilt is critical to the growth rate. The three kinds of derivatives of S, T, and density (Eulerian, Lagrangian, and synoptic along-core) are found to have different signs and phases, confounding attempts to confirm double diffusion by fine-scale observations, T-S diagrams in intrusion-affected regions can assume a variety of shapes. For a salinity perturbation amplitude S, the effective coefficients of horizontal diffusion can exceed S2 ⋅ 106 cm2/s for salt and heat, which is large enough to account for a significant part of the large-scale, nonadvective fluxes in the ocean. The vertical coefficients are greater than S2⋅103 cm2/s (S2⋅102 for density) and negative! The resulting strong positive feedback between large-scale gradients and medium-scale intrusions has widespread implications for the generation and maintenance of water mass properties.