B. Segment structure rules. SS-RULEs are a way of stating the features of classification that cut across the categories in the PS-RULEs. They allow the same category symbol to cover the different kinds and classes of the category. We will also use such rules to state the structural conditions that must hold for different categories to harmonize (belong to compatible classes). The left side of an SS-RULE may represent a terminal category. In this case the rule may subcategorize the category and the feature is distinguished by being confined in a light-orange box. The left side may refer to a particular feature. In this case the rule classifies the category and the feature is confined to a yellow box.