Decomposition of a digital curve into convex and concave parts is of relevance in several scopes of image processing. In digital plane convexity cannot be observed locally. It becomes an interesting question, how far one can decide whether a part of a digital curve is convex or concave by a method which is "as local as possible". In a previous paper, it was proposed to define the meaningful parts of a digital curve as meaningful parts of the corresponding polygonal representation. This technique has an approximative character. In our considerations, we use geometry of arithmetical discrete line segments. We will introduce an exact method to define convex and concave parts of a digital curve.