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 roundGarbanzo bean
1b.Bean elliptical or oblongGo to 2
2a.Bean whiteWhite northern
2b.Bean has dark pigmentsGo to 3
3a.Bean evenly pigmentedGo to 4
3b.Bean pigmentation mottledPinto bean
4a.Bean blackBlack bean
4b.Bean reddish-brownKidney 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 oblongGo to B
   B.  Bean has dark pigmentsGo to C
            C.  Bean color is solidGo to D
            C.  Bean color is mottledPinto bean
                     D.  Bean is blackBlack bean
                     D.  Bean is reddish-brownKidney bean
   B.  Bean is whiteWhite northern
A.Bean is roundGarbanzo 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



01aLeaves needle-likeGo to 02
01bLeaves flattened and scale-likeGo to 27
02aLeaves are in clustersGo to 03
02bLeaves are borne singlyGo to 15
03aTwo to five leaves in a clusterGo to 04  Genus Pinus
03bMore than five leaves in a clusterGo to 14
04aLeaves mostly 5 in a clusterWhite Pine (Pinus strobus)
04bLeaves 2 or 3 in a clusterGo to 05
05aLeaves mostly 3 in a clusterGo to 06
05bLeaves mostly 2 in a clusterGo to 08
06aLeaves twisted, less than 5 inches longPitch Pine (Pinus rigida)
06bLeaves straight, more than 5 inches longGo to 07
07aLeaves 5-10 inches long, cones very thornyLoblolly pine (Pinus taeda)
07bLeaves mostly over 10 inches long, cones unthornedLongleaf pine (Pinus palustris)
08aLeaves mostly longer than 3 inchesGo to 09
08bLeaves mostly shorter than 3 inchesGo to 11
09aLeaves rigid, bark grayishBlack pine (Pinus nigra)
09bLeaves narrower than 1.6mm; bark reddish brown or brownGo to 10
10aCones thornless, twigs brownNorway pine (Pinus resinosa)
10bCones thorny, twigs whitishShortleaf pine (Pinus echinata)
11aLeaves mostly wider than 1.5 mmGo to 12
11bLeaves mostly narrower than 1.5 mmGo to 13
12aLeaves mostly longer than 35 mmMugho pine (Pinus mugo)
12bLeaves mostly shorter than 35 mmJack pine (Pinus banksiana)

Twigs whitened

Virginia pine (Pinus virginiana)
13bTwigs not whitenedScotch pine (Pinus sylvestris)
14aLeaves deciduous, clusters of 20-40Larch (Larix sp.)
14bLeaves persistent, stiff, and four sidedTrue cedar (Cedrus sp.)
15aNeedles short and sharpGiant Sequioa  (Sequioadendron giganteum)
15bNeedles longer than 12 mmGo to 16
16aTiny pegs on twigsGo to 17
16bNo pegs on twigsGo to 22
17aPegs square, needles sharpGo to 18 Genus Picea
17bPegs round, needles flat and bluntHemlock (Tsuga sp.)
18aLeaves dark green or yellow greenGo to 19
18bLeaves blue-greenGo to 20
19aBranchlets droopNorway spruce (Picea abies)
19bBranchlets do not droopRed spruce (Picea rubens)
20aLeaves at right angles to stemsBlue spruce (Picea pungens)

Leaves point forward

Go to 21
21aLeaves about 12 mm long, seed cones 15-32 mm in length, crown narrow and pointedBlack spruce (Picea mariana)
21bLeaves about 19 mm long, seed cones 50 mm in length, spire-like crown

White spruce (Picea glauca)

22aBuds large and pointedDouglas fir (Pseudotsuga sp.)
22bBuds small and roundedGo to 23
23aTerminal buds round and clusteredTrue fir (Abies sp.)
23bTerminal buds not clusteredGo to 24
24aNeedles white underneathGo to 25
24bNeedles green underneathGo to 26  Genus Taxus
25aNeedles pointed

Redwood (Sequoia sempervirens)

25bNeedles bluntHemlock (Tsuga sp.)
26aLeaves 18 mm long or less with inconspicuous midribAmerican Yew (Taxus canadensis)
26bLeaves 25 mm long or more with conspicuous midribJapanese Yew (Taxus cuspidata)
27aAll leaves short and sharpGiant Sequioa  (Sequioadendron giganteum)
27bSome leaves not sharpGo to 28
28aCones roundGo to 29
28bCones not roundGo to 31
29aCones soft and leatheryJuniper (Juniperus sp.)
29bCones woodyGo to 30
30aCones under 12 mm in diameterFalse cypress  (Chamaecyparis)
30bCones over 12 mm in diameterCypress (Cuppressus)
31aCones resemble rosebudsWhite cedar or arbor vitae (Thuja)
31bCones resemble duck billsIncense 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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