Seven models relating similarity in pairs of two-dimensional stimuli are stated; three derive from distance measures and four from so-called content models. The metatheoretical status of assumptions underlying the various models is outlined. Two experiments, one using Dot pattern stimuli and the other Brick Wall patterns, on 54 and 50 subjects respectively, were chosen for their relation to previously published work and analysed using all of the seven models. A normed distance model, which can be equivalently expressed as a content model but not as a Minkowski distance model, was found to be marginally superior to alternatives. Under some conditions the predictions of the alternative models are practically indistinguishable, but a Euclidean distance model is inferior in this context. Implications for scaling and modelling are noted.