In my July 2001 column, "Are You in Tune?" (InstantDoc ID 21038), which began a series about query tuning, I mentioned that one of the best ways to ensure that your queries are running as fast as possible is to check that your tables are properly indexed. However, I didn't discuss index structures in depth. A clear understanding of how SQL Server organizes indexes can be a big advantage in determining what kind of indexes can be useful for which queries. This month, I look at how the information in indexes is organized. And I discuss some of the tools you can use to examine the structures of your indexes and gain an even deeper understanding of the valuable information they contain.