A wood species must be in the same genus in order to be considered related in this field. Hence, some woods that seem to be related, such as African Mahogany and Honduran Mahogany, will not be listed because they belong to different genera (Khaya and Swietenia, respectively). Likewise, other woods that seem completely unrelated, such as Narra and Padauk, will show up as related species since they are both from the same genus (Pterocarpus).
Some exceptions to this rule are made: for instance, if two woods are very similar in appearance and working properties, and are in closely related genera within the same family, they may be listed as related. An example of this is Lignum Vitae and Argentine Lignum Vitae, which are in different genera, but both belong to the Zygophyllaceae family.