A key is a device for easily and quickly identifying an unknown organism. The dichotomous key is the most widely used type in biological sciences. The user is presented with a sequence of choices between two statements, couplets, based on characteristics of the organism. By always taking the correct choice, the name of the organism will be revealed.

Two ways to set up a dichotomous key:

1. Present each couplet together: Although the choices are easy to see in this key, relationships between various characteristics are not emphasized.

1a. Fruit is a group of akenes........................go to 2
1b. Fruit is a group of follicles........................go to 4

2a. Petals none............................................go to 3
2b. Petals present.........................................Ranunculus

3a. Sepals usually 4......................................Clematis
3b. Sepals usually 5......................................Anemone

4a. Flowers regular........................................Aquilegia
4b. Flowers irregular......................................Delphinium

2. Group by relationships: This key has exactly the same choices as the first example. The choices are separated, but it is easy to see the relationships. While this key might be more difficult to construct, it gives more information to the user. For example, in the key below it is easy to see that the fruit of Aquilegia and Delphinium is a follicle. This is not as obvious in the first type of key.

1a. Fruit is a group of akenes..................................go to 2
2a. Petals none............................................go to 3
3a. Sepals usually 4............................Clematis
3b. Sepals usually 5............................Anemone
2b. Petals present.........................................Ranunculus
1b. Fruit is a group of follicles..................................go to 4
4a. Flowers regular........................................Aquilegia
4b. Flowers irregular......................................Delphinium

In constructing keys, keep the following in mind:

• Use constant characteristics rather than variable ones.
• Use measurements rather than terms like "large" and "small".
• Use characteristics that are generally available to the user of the key rather than seasonal characteristics or those seen only in the field.
• Make the choice a positive one - something "is" instead of "is not".
• If possible, start both choices of a pair with the same word.
• If possible, start different pairs of choices with different words.
• Precede the descriptive terms with the name of the part to which they apply.

When using a key, keep the following in mind:

• Always read both choices, even if the first seems to be the logical one at first.
• Be sure you understand the meaning of the terms involved. Do Not Guess.
• When measurements are given, use a calibrated scale. Do Not Guess.
• Since living things are always somewhat variable, do not base your conclusion on a single observation. Study several specimens to be sure your specimen is typical.
• If the choice is not clear, for whatever reason, try both divisions. If you end up with two possible answers, read descriptions of the two choices to help you decide.
• Having arrived at an answer in a key, do not accept this as absolutely reliable. Check a description of the organism to see if it agrees with the unknown specimen. If not, an error has been made somewhere, either in the key or in its use. The ultimate check of identifications is a comparison of the unknown with an authentically named Type Specimen.