When you create a clustered index, all rows in the table are sorted and stored in the clustered index key order. Because the rows are physically sorted by the index key, you can have only one clustered index per table. You can think of the structure of a clustered index as being similar to a filing cabinet: The data pages are like folders in a file drawer in alphabetical order, and the data rows are like the records in the file folder, also in sorted order.
You can think of the intermediate levels of the index tree as the file drawers, also in alphabetical order, that assist you in finding the appropriate file folder. Figure 31.16 shows an example of a clustered index tree structure.