Optical coupling, or crosstalk, between two parallel, identical, single-mode fibres is describable as a superposition or the ψ+ and ψ− modes of the composite waveguide. Perturbation theory gives ψ+ and ψ− as the sum and difference, respectively, of the fundamental modes ψ1 and ψ2 of each fibre. In reality ψ+ is the fundamental mode of the composite waveguide, whereas ψ− is the second mode and has a finite cutoff. This limits the smallest V-value for which low-loss coupling is possible. Exact values and approximate analytical expressions for cutoff are presented and compared with experiment. The limitation is least severe for step-profile fibres.