Types of identification keys

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There are several types of identification keys. The more traditional and probably the most common type is the single-access key, but multi-access and tabular keys are often more useful.

Single-access key

The classical key in biology is a single-access key (also called a “branching key”). In this key the sequence and structure of identification steps is fixed by the author of the key. At each point in the decision process, multiple alternatives are offered, each leading to a result or a further choice. The alternatives are commonly called "leads", the set of leads at a given point a "couplet". If the entire key consists of exactly two choices at each branching point, the key is called dichotomous, otherwise it is described as polytomous. The majority of single-access keys are dichotomous.

Single-access keys have been in use for several hundred years. They may be printed in various styles (e. g., linked, nested, graphical) or used as interactive, computer-aided keys.

An example of a linked dichotomous key.
  • Linked style has each couplet numbered and all leads of a couplet immediately follow each other. A pointer linking to the resulting couplet, taxon, or subkey is given at the end of each lead. This style is more suitable for long keys and polytomous keys, because finding and comparing the corresponding leads in a couplet is more convenient and reliable. Compared with nested keys using indentation, it makes better use of available space.
An example of a nested dichotomous key.
  • Nested style has split leads within a couplet and all couplets that logically follow a given lead also follow immediately in the display sequence. These couplets are usually indented. Taxon or subkey results follow at the end of leads as in the previous style, but pointers to the next couplet are redundant (except for redirections). The major advantages of this style is that the resulting groups can clearly be seen and that it is much easier for experts to make use of additional knowledge about the species being keyed out. This style is often preferred by taxonomists who wish to express taxonomic structure directly in the key. In general, the nested style is recommendable mostly for short keys of less than a page in print.
An example of a graphic dichotomous key.
  • Graphical style is a rare but very appealing presentational form where the graphical elements and illustrations dominate the layout and key structure. This style, with a combination of text and illustrations, is common in simple keys for primary school children, but it is useful also for older students and experts. It is very space-consuming and is best used for short keys (one page in print).

In the interactive (computer-aided) keys, either a longer part of the key may be displayed (optionally hyperlinked), or only a single question may be displayed at a time.

More about single-access keys on Wikipedia
Example of a hyperlinked key Key to the garden and village birds of Europe
Examples of non-hyperlinked, indented nested keys Key to the aquatic plants of British Columbia
Example of a non-hyperlinked, graphical key Key to the woodlice that you are most likely to find in Great Britain.

Muliti-access keys

Part of an interactive multi-entry key.

If the information for a given identification step in a single-access key is not available, several potential leads must be followed and identification becomes increasingly difficult. This problem is solved with multi-access keys which allow free choice of identification steps.

A multi-access key allows the user to adapt the key to the particular organism that is being identified and to the circumstances of identification (e.g. field or laboratory). This is very useful as those characters which are necessary for identification are often not all obtainable at a given time for every taxon in a key (especially in keys for identifying plants).

Multi-access keys may be printed in various way (tabular, matrix, formula style, etc.) but are more commonly used as computer-aided, interactive keys. In a typical multi-access key the choice of characters used for identification can be repeated multiple times (reducing the number of remaining taxa each time) while in multi-entry keys the free choice of characters is allowed only in the first step. Examples for multi-entry keys are those created by the FRIDA software, where a single step of selecting one or multiple criteria is followed by a dichotomous key for the species remaining after this step.

More about multi-access keys on Wikipedia
Example of a hyperlinked key guide to lichens on twigs
Example of an interactive key British bumblebee identification guide (Flash version)
Several multi-entry keys are available in the pages of the Dryades project Dryades, section identification tools.

An example of a tabular key.
Tabular keys

A tabular key is a special key style combining properties of branching and multi-access keys.

The tabular arrangement is very space-consuming so that only a few characters can be arranged in this manner. Usually the entire key is therefore split into many small subkeys, and characters specific to the differentiation of certain taxa may be presented in the form of natural language descriptions rather than in separate columns.

Tabular keys are found both in print and as hyperlinked text on the web. They may include illustrations, either through reference, or sometimes directly within the table cells.

Examples of hyperlinked, tabular keys Wood types