Many subtleties related to the form of type descriptions is perhaps evident from the examples given above. Then again, perhaps not! It is useful to think of type descriptions as having many different 'depths', loosely corresponding to the number of bonds followed away from a central atom (the target atom, or the one currently being tested against the type description). The target atom of the type description is the 'root' of the description since all connections are defined relative to this atom. A type description requiring specific connections to this target atom is using the target atom's bonds in order to identify it - atoms to a depth of 1 bond are being used to describe the atom. If these bound atoms are in turn described by their bound neighbours then atoms to a depth of 2 bonds are being used.

For example:

Any depth of description can be handled by Aten, becoming as complex as is necessary to uniquely identify the target atom.