CONSTRUCTING A DICHOTOMOUS KEY

MARGARET BANKHEAD              ROBERT A. BLACK MAGNET SCHOOL
                               9101 SOUTH EUCLID AVE.
                               CHICAGO, IL 60649
                               1-312-375-2040

Objectives:
     To construct and use dichotomous keys.

Materials:
     Shoes (Use one shoe from each of 10 students).
Worksheets:  "Using A Dichotomous Key"
             "Imaginary Animals"


Strategies:
Procedure:
1.Arrange chairs in a circle.
2.Instructor takes off one shoe and place it on the floor in the middle of 
the circle.

Dialogue
3."There's one of my shoes, so let's have a shoe from about ten of you".
4.Tell the students that they are to divide the shoes into two piles. Tell 
them the piles don't have to have equal numbers of shoes but, that they all 
have to agree on some obvious characteristic that will distinguish the shoes 
in one pile from the shoes in the other pile.After agreement is reached,tell 
the students a record of the agreement will be kept on the board.
5.Draw two horizontal lines some distance apart on the chalkboard. Label the 
lines with the agreed upon characteristics. (Fig.1).
 
             __________            

             __________
             FIG.1


6.Return to the pile of shoes. Tell the students one pile will be pushed 
aside for the moment but, now they must again divide the pile of shoes into 
two distinct piles. After agreement is reached add this information to the 
chalkboard sketch. (Fig.2).

                       _____________
       _______________/ 
                      \_____________ 

                      FIG.2

7. Continue the procedure of dividing the shoes into two distinct piles and 
adding the information to the sketch until there is only one shoe with the 
identifying characteristic, at which point the shoe is identified and the 

owners name is added to the sketch (Fig.3).              ______________
                                           _____________/    
                             _____________/             \______________
                                          \_____________ 
                                                         
                             FIG. 3                            

8. Divide the second pile of shoes in the same manner as the first pile 
until all the shoes have been identified.
9. Push all the shoes back together in one pile[Adding perhaps a shoe from 
the distance past or one with characteristics unlike those used in the 
original construction].
 Discuss the meaning of the term dichotomous explaining that the word 
means "two forks".
10. Tell the students that dichotomous keys usually appear in a more 
compact form and that the diagram can be easily converted by adding numbers 
to each characteristic used. Label the diagram in numerical sequence 
following the same order the characteristics were agreed upon.
11. Have the students redeem their shoe by taking it from the pile and 
placing it on the correct branches of the key which will lead to its'  
correct identification.  After all the shoes have been redeemed the shoe 
added earlier should remain. Ask a student to follow the key until the shoe 
is identified. Students should discover that a key works only for 
identification of those items used in its' original construction.   
12. Pass out the two worksheets. Complete the " Using A Dichotomous Key" 
sheet in class. Assign the " Imaginary Animals" sheet for homework.