A build tree, as defined before, is a collection of build items arranged hierarchically in the file system. Like build items, build trees have names, and are only referred to from other build trees by name. The root of a build tree is a build item whose Abuild.conf contains the tree-name key. We refer to this item as the tree's root build item.

A build tree is formed as a result of the items it contains holding references to the locations of their children within the file system hierarchy. These locations are named as relative paths in the child-dirs keys of the items' Abuild.conf files. It is customary to have the value of child-dirs contain single path elements (i.e.just a directory without any subdirectories), but this is also not a requirement: child-dirs entries may contain multiple path elements as long as there are no Abuild.conf files in any of the intermediate directories. If a build item's child contains its own tree-name key, that child build item is the root of a separate build tree that is part of the same forest, defined below. Otherwise, the child build item is part of the same tree as its parent.

In addition to containing build items, build trees can contain other attributes. Among these are references to other build trees, a list of supported traits, and a list of plugins. We will discuss these topics later in the document. These attributes are defined using keys in the root build item's Abuild.conf file.