Dichotomous Keying


The identification of biological organisms can be greatly simplified using tools such as dichotomous keys.  A dichotomous key is an organized set of couplets of mutually exclusive characteristics of biological organisms.  You simply compare the characteristics of an unknown organism against an appropriate dichotomous key.  These keys will begin with general characteristics and lead to couplets indicating progressively specific characteristics. If the organism falls into one category, you go to the next indicated couplet.  By following the key and making the correct choices, you should be able to identify your specimen to the indicated taxonomic level. 

Couplets can be organized in several forms.  The couplets can be presented using numbers (numeric) or using letters (alphabetical).  The couplets can be presented together or grouped by relationships.  There is no apparent uniformity in presentation for dichotomous keys.

Sample keys to some common beans used in the kitchen:

Numeric key with couplets presented together.  The major advantage of this method of presentation is that both characteristics in a couple can be evaluated and compared very easily.



Bean round

Garbanzo bean


Bean elliptical or oblong

Go to 2





Bean white

White northern


Bean has dark pigments

Go to 3





Bean evenly pigmented

Go to 4


Bean pigmentation mottled

Pinto bean





Bean black

Black bean


Bean reddish-brown

Kidney bean

Alphabetical key with couplets grouped by relationship.  This key uses the same couplet choices as the key above.  The choices within the first and succeeding couplets are separated to preserve the relationships between the characteristics.



Bean elliptical or oblong

Go to B


   B.  Bean has dark pigments

Go to C


            C.  Bean color is solid