Couplets, Targets and Rows

Before using FNA keys to identify a specimen, the family must be known. If the family is not known, use, for example, the angiosperm and gymnosperm key to families, which are multi-access keys, created as part of the Pathkey project.

In each FNA key, couplets are numbered starting with 1.  A couplet gives a choice of two alternatives, each of which describe one or more characteristics (called "characters") that are to be matched with the specimen that is being identified. The first choice in the key 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, which is a taxon with rank normally one lower than the rank of the key.

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 to species using couplet numbers and destination targets:

As a choice is made at each couplet, the characters set (see Characters Sets and Paths) for the target grows.

When creating a target's characters set, it is easier to use the key's row numbers as an index into the key, instead of using couplet numbers. The keys are laid out such that the row number can be obtained from the couplet number:

Couplet number	Row number for current couplet
	Using first choice	Using second choice
n	2n - 1	2n

In other words, the row number of the second choice is double the couplet number, and the row number of the first choice is one less than this. This assumes that each couplet lead is only on one row; that is all lines for characters of a lead are put into one row. The algorithms for traversing the keys depend on this couplet-to-row relationship; see Key Types and Subkeys and Relation of Key Structure to Taxonomic Hierarchy for how special cases are handled to maintain this relationship.

This is another version of the diagram for the Caryophyllaceae Drymaria key using row numbers instead of couplet numbers:

A row number 0 (representing the root of the Drymaria key) has been added as a starting point.

When row numbers are needed, a third version that has both couplet and row numbers is easier to use. For an example of such a diagram, see the one in Characters Sets and Paths for Brassicaceae Boechera. Notice in that diagram when the row numbers are twice couplet numbers. Also note that 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.

Couplet diagrams are easier to create and understand, but rows are used to help create the characters-set list for a target, as described in Characters Sets and Paths and are needed to understand how keys at different ranks are connected, as described in Key Structure.

As discussed in Key Types and Subkeys, when keys for the multiple genera of a family are concatenated to form the key to species for the family, rows are renumbered, so that the row number applies to the key for the family. That is, for the family key, couplet numbers are less useful because they only apply to the key for a given rank, but row numbers can be used to uniquely identify any couplet or target in the key at any rank in the family. Also row numbers are used in constructing nodes, as discussed in Node Structure and Types.