Component compostions in the construction of the HDT. The two different ways of constructing a C component by merging two underlying components. The topmost of the components can either be a C component (a) or an I component (b) while the bottommost component, C1, must be a C or L component. If the topmost component is an I component, the bottommost must be downwards closed, i.e. it cannot have a downwards edge crossing its boundary.