"These species codes, having six letters rather than the four letters of more commonly used systems, almost read themselves, which is not usually the case for four-letter codes. Also, there are many fewer special cases necessitated by system codes being identical for two species. The simple rules given below allow construction of species abbreviations. Note that these rules do not include exceptions such as always using BK for "black" and "BL" for "blue", etc. Including exceptions would make it necessary for the user of the system to remember these exceptions both in writing codes and reading the codes. This would defeat the purpose of having a rule-based system." [Bruce M. Bowman]
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