Charge orders in the kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ sit at the center of a rich phase diagram that also includes superconductivity, nematicity, and signatures of time-reversal-symmetry breaking. Yet even the basic question of which charge-ordering wavevectors are intrinsic, and which are selected by dimensionality and lattice coupling, remains unsettled. Importantly, the microscopic origin of different charge orders and, in particular, the relationship between the robust bulk $2\times 2$ charge order and the controversial $4\times 1$ modulation, which is primarily resolved by surface probes, remains unresolved. Here, we use first-principles calculations to study the role of interlayer coupling in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ by tuning the interlayer separation from the monolayer limit to the bulk limit. In the monolayer $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = Rb, Cs), the phonon spectrum exhibits no instability at the M point; instead, the dominant lattice instability occurs at $\text{q}=(1/4,0,0)$, consistent with a $4\times 1$ modulation. As interlayer coupling increases in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, an M-point phonon progressively softens and becomes unstable already near $c\approx 12.24\AA$, evolving into the strong $2\times 2$ instability characteristic of the bulk. These results identify interlayer coupling as a control parameter at a fixed stoichiometry that links competing $4\times 1$ and $2\times 2$ tendencies, providing a unified framework for understanding why multiple charge-order wavevectors coexist and compete in kagome metals.

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• Charge density waves
• Materials modeling