In each FNA key for a given taxon, couplets are numbered starting with 1. A couplet gives a choice of two alternate attributes: the first choice is labeled with the couplet number and the second is labeled "+". Following each choice where to go next in the key is indicated by a couplet number. However instead of a next couplet, one or both choices may lead to a destination target.
A diagram showing the hierarchy of couplet numbers is a simple way to show the overall structure of a key. For example, the following diagram shows the structure of the Caryophyllaceae Drymaria
key using couplet numbers and destination targets:
Each choice in a couplet has one or more attributes that are added to the target's attribute set.
When creating a target's attribute set, it is easier to use the row numbers as an index into the key, instead of using couplet numbers. The keys are designed such that the row number can be obtained from the couplet number:
||Row number for current couplet
|Using first choice
||Using second choice
||2n - 1
In other words, the row number is double the couplet number, except for the first choice, the row number is one less than this. See the last diagram in this section as an example of how row numbers (in circles) are related to couplet numbers; note in that diagram, the row number of the first choice in a couplet is put to the left of the couplet, and the second choice is put to the right.
Two FNA keys use a special structure exception that must be handled correctly so that the above relationship between couplet and row numbers is maintained. In particular, the Ranunculus and Piperia keys to species have targets that are, instead, subspecies. [Need to a pointer to some later post on implementation details that has a diagram showing an example of having subspecies target nodes and special species nodes added after the key nodes to connect these together: e.g. diagram in 8/26/15 notes.]
This is a diagram of the Caryophyllaceae Drymaria key using row numbers:
A row number 0 (representing the root of the Drymaria key) has been added as a starting point. From this diagram, you can create the path to any row in the key or to any target; in particular, you can create the attribute set for any target. Also it is useful to create a mapping of any row to its parent row.
Except these uses, a row diagram of a key has disadvantages: it is harder to create and understand; so a couplet diagram is used most often. A diagram that has both may be a better choice when row numbers are needed. For example, the following diagram for the Brassicaceae Boechera key
can be used to create the paths to Boechera repanda, Attribute Sets 1 and 2:
With the row numbers, you can create the lists of the common attributes and specific attributes for sets 1 and 2, which are shown in the Attribute Sets
post. How the subkeys in the Boechera key are combined is discussed at the end of the Key Types and Subkeys
post; in particular, the Group 1 subkey is used in this diagram.