Under Tapestry, it is common to define new components by combining existing components. The existing components are embedded in the containing component. This is always true at the top level; Pages, which are still Tapestry components, always embed other Tapestry components.

Each embedded component has an id (an identifying string) that must be unique within the containing component. Every non-page component is embedded inside some other component forming a hierarchy that can get quite deep (in real Tapestry applications, some pages have components nested three to five levels deep).

In some cases, a component will be referenced by its id path. This is a series of component ids separated by periods, representing a path from the page to a specific component. The same notation as a property path is used, but the information being represented is quite different.

For example, the id path border.navbar.homeLink represents the component named homeLink, embedded inside a component named navbar, embedded inside a component named border, embedded inside some page.

Tapestry components are "black boxes". They have a set of parameters that may be bound, but their internals, how they are implemented, are not revealed.

Primitive components may not embed other components, or even have a template. Nearly all the built-in components are primitive; they are building blocks for constructing more complex components.

Alternately, a component may be implemented using a template and embedded components. In either case, the names, types or very existence of embedded components is private, hidden inside the containing component's "black box".