Dichotomous Keying


Dichotomous Keying

Introduction to Dichotomous Key Maker:

The identification of biological organisms can be greatly simplified using tools such as dichotomous keys.  A dichotomous key maker 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.





1a. Bean round Garbanzo bean
1b. Bean elliptical or oblong Go to 2
2a. Bean white White northern
2b. Bean has dark pigments Go to 3
3a. Bean evenly pigmented Go to 4
3b. Bean pigmentation mottled Pinto bean
4a. Bean black Black bean
4b. 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.



A. Bean elliptical or oblong Go to B
   B.  Bean has dark pigments Go to C
            C.  Bean color is solid Go to D
            C.  Bean color is mottled Pinto bean
                     D.  Bean is black Black bean
                     D.  Bean is reddish-brown Kidney bean
   B.  Bean is white White northern
A. Bean is round Garbanzo bean


Rules for Using Dichotomous Keys: 

When you follow a dichotomous key, your task becomes simpler if you adhere to a few simple rules of thumb:

  1. Read both choices in a couplet carefully.  Although the first description may seem to fit your sample, the second may apply even better.
  2. Keep notes telling what sequence of identification steps you took.  This will allow you to double-check your work later and indicate sources of mistakes, if they have been made.
  3. If you are unsure of which choice to make in a couplet, follow both forks (one at a time).  After working through a couple of more couplets, it may become apparent that one fork does not fit your sample at all.
  4. Work with more than one sample if at all possible.  This will allow you to tell whether the one you are looking at is typical or atypical.  This is especially true when working with plants – examine more than one leaf, branch, cone, seed, flower,…etc.
  5. When you have keyed out an organism, do not take your effort as the final result.  Double check your identification scheme, using your notes.  Find a type specimen (if available) and compare your unknown to the type specimen.  If a type specimen is unavailable, find a good description of the indicated taxonomic group and see if your unknown reflects this description.
  6. When reading a couplet, make sure you understand all of the terms used.  The best keys will have a glossary of technical terms used in the key.  If a glossary is unavailable, find a good reference work for the field (textbook, biological dictionary,…etc.) to help you understand the term.
  7. When a measurement is indicated, make sure that you take the measurement using a calibrated scale.  Do not “eyeball” it or take a guess.

Exercise 1:

Using a container of beans, use one of the dichotomous keys above to identify the beans.  Glue the beans to the card provided and label them with their common name. Indicate what steps you followed to arrive at your answer.  Turn the card in to your instructor.  Compare your answers to the instructor’s descriptions and type specimen.

Exercise 2:

Obtain samples of the snack chips provided.  Develop a dichotomous key to identify the snacks.  In your notebook, keep track of the characteristics you used to differentiate between the different snack families.  What are the values of the characteristic for each snack food?

Exercise 3:

Use the dichotomous key to conifers provided below to identify conifers.

A Key to Selected North American Native and Introduced Conifers



01a Leaves needle-like Go to 02
01b Leaves flattened and scale-like Go to 27
02a Leaves are in clusters Go to 03
02b Leaves are borne singly Go to 15
03a Two to five leaves in a cluster Go to 04  Genus Pinus
03b More than five leaves in a cluster Go to 14
04a Leaves mostly 5 in a cluster White Pine (Pinus strobus)
04b Leaves 2 or 3 in a cluster Go to 05
05a Leaves mostly 3 in a cluster Go to 06
05b Leaves mostly 2 in a cluster Go to 08
06a Leaves twisted, less than 5 inches long Pitch Pine (Pinus rigida)
06b Leaves straight, more than 5 inches long Go to 07
07a Leaves 5-10 inches long, cones very thorny Loblolly pine (Pinus taeda)
07b Leaves mostly over 10 inches long, cones unthorned Longleaf pine (Pinus palustris)
08a Leaves mostly longer than 3 inches Go to 09
08b Leaves mostly shorter than 3 inches Go to 11
09a Leaves rigid, bark grayish Black pine (Pinus nigra)
09b Leaves narrower than 1.6mm; bark reddish brown or brown Go to 10
10a Cones thornless, twigs brown Norway pine (Pinus resinosa)
10b Cones thorny, twigs whitish Shortleaf pine (Pinus echinata)
11a Leaves mostly wider than 1.5 mm Go to 12
11b Leaves mostly narrower than 1.5 mm Go to 13
12a Leaves mostly longer than 35 mm Mugho pine (Pinus mugo)
12b Leaves mostly shorter than 35 mm Jack pine (Pinus banksiana)

Twigs whitened

Virginia pine (Pinus virginiana)
13b Twigs not whitened Scotch pine (Pinus sylvestris)
14a Leaves deciduous, clusters of 20-40 Larch (Larix sp.)
14b Leaves persistent, stiff, and four sided True cedar (Cedrus sp.)
15a Needles short and sharp Giant Sequioa  (Sequioadendron giganteum)
15b Needles longer than 12 mm Go to 16
16a Tiny pegs on twigs Go to 17
16b No pegs on twigs Go to 22
17a Pegs square, needles sharp Go to 18 Genus Picea
17b Pegs round, needles flat and blunt Hemlock (Tsuga sp.)
18a Leaves dark green or yellow green Go to 19
18b Leaves blue-green Go to 20
19a Branchlets droop Norway spruce (Picea abies)
19b Branchlets do not droop Red spruce (Picea rubens)
20a Leaves at right angles to stems Blue spruce (Picea pungens)

Leaves point forward

Go to 21
21a Leaves about 12 mm long, seed cones 15-32 mm in length, crown narrow and pointed Black spruce (Picea mariana)
21b Leaves about 19 mm long, seed cones 50 mm in length, spire-like crown

White spruce (Picea glauca)

22a Buds large and pointed Douglas fir (Pseudotsuga sp.)
22b Buds small and rounded Go to 23
23a Terminal buds round and clustered True fir (Abies sp.)
23b Terminal buds not clustered Go to 24
24a Needles white underneath Go to 25
24b Needles green underneath Go to 26  Genus Taxus
25a Needles pointed

Redwood (Sequoia sempervirens)

25b Needles blunt Hemlock (Tsuga sp.)
26a Leaves 18 mm long or less with inconspicuous midrib American Yew (Taxus canadensis)
26b Leaves 25 mm long or more with conspicuous midrib Japanese Yew (Taxus cuspidata)
27a All leaves short and sharp Giant Sequioa  (Sequioadendron giganteum)
27b Some leaves not sharp Go to 28
28a Cones round Go to 29
28b Cones not round Go to 31
29a Cones soft and leathery Juniper (Juniperus sp.)
29b Cones woody Go to 30
30a Cones under 12 mm in diameter False cypress  (Chamaecyparis)
30b Cones over 12 mm in diameter Cypress (Cuppressus)
31a Cones resemble rosebuds White cedar or arbor vitae (Thuja)
31b Cones resemble duck bills Incense cedar (Calocedrus)


Conifers to Identify:

1. Name: 2. Name:

3. Name: 4. Name:

5. Name: 6. Name:

7. Name: 8. Name:

9. Name: 10. Name:

11. Name: 12. Name:

13. Name: 14. Name:

15. Name: 16. Name:

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