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Displaying hierarchical content is a common request of database-driven web sites. Drupal gives you great flexibility in doing this without writing a single line of code, using a couple of battle-tested modules. As an example, let's say you're building an automotive web site. Your site is going to have a list of automotive brands and the vehicles they build. You'd like to only have to enter each brand's information into the database once, then simply refer to that brand whenever you add one of their vehicles to the site. In this example, I have 2 brands, Chevy and Jeep, and each brand has 3 vehicles.

Drupal's powerful CCK module allows you to define different content types for your web site. The Node Reference module, included with the rest of CCK, allows you to set up hierarchical relationships between content types.