To further our understanding of how complex anisotropic structure–property relationships may be rationalized by their local atomic arrangements in ferroelectric materials, using the newly found metastable monoclinic Pm phase of potassium niobate (KNbO₃) as an example, we perform first-principles density-functional (perturbation) theory calculations to understand how applied epitaxial strain may influence their structural, thermodynamic, electronic, and (anisotropic) polarization properties in polar KNbO₃ polymorphs – a potential contender for Pb-free piezoelectric applications. Here, we find that the displacement of the center metal cation (niobium, Nb) relies on more complex anisotropic properties than the commonly used isotropic scalar quadratic elongation, 〈λ〉 for the monoclinic Pm phase, showing an anisotropic nonlinear relationship between ε_gap and 〈λ〉. We also show how anisotropic ferroelectric distortion under strain may strongly influence the direction-dependent chemical bonding character in monoclinic KNbO₃. Lastly, building on the isotropic 〈λ〉 index, we propound a revised definition of this key structural descriptor – the modified bond elongation index (_i), which contains vectorial structural information. Using _i, we successfully rationalize and demonstrate the linear dependency of direction-dependent P_s on _i for strained KNbO₃ polymorphic phases.