A parts structure can be thought of as a family tree showing the hierarchical relationships of all the parts. Because of this hierarchical nature, we talk about the different levels of the structure. At the highest level a single part exists; this is the Top part already discussed. For your first few drawings using parts, it is best to use a parts structure with only one level; more experienced users will use a multi-level structure. The two types of parts structure are shown in the example below: