Top: UCAC4 proper motions for NGC 2360. Bottom left: TGAS proper motions for the same set of stars, with nominal uncertainties represented as error bars. Bottom middle: same as bottom left panel, but the uncertainties are represented as non-covariant error ellipses. Bottom right: same data as bottom middle panel, with uncertainties represented as tilted error ellipses, showing the correlations between μ_α * and μ_δ errors (here of the order of −0.8).