The researchers introduced into the experiment the concept of the interface, or allowed boundary over which character states would flow. A one-way interface, or edge, existed between a parent and a child. If only one-way edges were sufficient to explain the presence of all the states in a language, then there was no need to look beyond the perfect phylogeny. If not, then one or more contact edges, or bidirectional interfaces, could be added to the phylogeny. A language therefore might have more than one source of states: the parent or a contact language.