The vibration response of a small harp is studied, by a combination of measurement, theoretical modelling and model-based sound synthesis. The harp's distinctive angle between strings and soundboard is shown to have little impact on the results of linear modelling, but to have a significant effect on sound through a nonlinear phenomenon. Generation of axial force in the string through geometric nonlinearity is shown to produce very prominent effects, helped by the efficient coupling of longitudinal force to soundboard motion. The detailed pattern of the resulting “phantom partials”, already familiar from work on piano strings, is shown to be sensitive to the exact boundary condition between the string and the soundboard: better agreement between experiment and modelling is obtained by assuming a clamped boundary than a pinned boundary. It is suggested that the relative prominence of nonlinear effects in the harp, compared to other plucked-string instruments using non-metallic strings, may be one reason why harpists still prefer the sound of gut strings over nylon strings.