This paper proposes a structural description scheme using second order primitives, in particular circular arcs. In this framework, a distance measure between pairs of circular arcs and relations among them is introduced, and its main properties are discussed. This measure accomplishes some perceptive criteria for increasing its efficiency: it proved applicable in a wide class of application domains characterised by high variability in the shape of the visual patterns, where a structural approach is particularly useful. The description method together with the distance have been experimentally validated in the context of the recognition of handwritten digits coming from a standard character database.