There are two different kinds of pure menu systems - the pure "drop down" menu system and the pure "slide right" menu system. In a pure "drop down" system, the menus all have a horizontal/row orientation and an expanded item appears below its parent. In a pure "slide right" system, the menus all have a vertical/column orientation and an expanded item appears to the right of its parent. They are illustrated on the following page: menus-pure.html.

This page illustrates a "traditional" menu system that uses a hybrid approach in which the first-level menus "drop down" (but with a vertical/column orientation) and subsequent levels "slide right".

You can see this by clicking on the [Developer] and [Developer]-[Encoding] items, both of which expand.