In this paper a method of understanding of the concave polygon object is presented. The proposed method of understanding the concave polygon object is part or the shape understanding method. The main novelty of the presented method is that the process of understanding of the concave polygon object is related to the visual concept represented as a symbolic name of the possible classes of shape. The possible classes of shape, viewed as hierarchical structures, are incorporated into the shape model. At each stage of the reasoning process that led to assigning of an examined object to one of the possible classes, the novel processing methods were used. These methods, implemented as modules of the shape understanding system (SUS) and tested on the broad classes of shapes, are very efficient because they deal with a very specific classes of shape. The system consists of different types of experts that perform different processing and reasoning tasks. The concave polygon class models and processing methods are also described. (c) 2005 Elsevier Ltd. All rights reserved.