1. Creating a
Dichotomous Key
© S. Tanner / 1999

2. What is a Classification Key?
Classification keys are used to identify an unknown biological specimen.
One of the most frequently used types of keys is the “Dichotomous Key”
What does “Dichotomous” mean?

3. Dichotomous
“Di”
(two)
“Chotomy”
(choices)

4. A dichotomous key always presents you with two choices
Two choices of what?
The choices are just different characteristics, or features, of the specimen. For example, is the shape of a leaf tip pointed or round?

5. Or, for the characteristic “leaf type”, is the leaf...
A broadleaf type
Or needlelike?

6. It’s a little tricky to make a dichotomous key for another person to use, but there is a process you can follow to develop a key that works well.

7. Start by gathering a sample set of specimens

8. You will create a key for a group of seven specimens
You will create a key for a group of seven specimens. They are all broadleafed deciduous trees (they lose their leaves at the end of each growing season).

9. Look at the sample specimens shown on the next slide
Look at the sample specimens shown on the next slide. Study them carefully, then write a list of some of the general leaf characteristics that could be used to discuss differences between the specimens.
Note:
These should just be general features, not the specific descriptions you will use in your dichotomous key.

10. general
features?

11. How many of the following leaf characteristics did you come up with?
Leaf arrangement
Leaf shape
Margin (edge) features
Tip shape
Vein pattern
Length of stalk (stem)

12. There are two general types of leaf arrangements:
Single leaves - one leaf
on each leaf stalk.
Compound leaves -
multiple leaves on one
leaf stalk.

13. Here are typical descriptions of the different leaf shapes:

14. These are the different leaf margin (edge) features:

15. Leaf tip shapes:

16. Leaf vein patterns:

17. Leaf stalk lengths:

18. There are other features of trees that could have been used if you had more information about the trees, for example:
bark features
tree silhouette shape
seed and cone characteristics

19. Tree silhouette shapes:
Tree seeds
and cones:

20. Now it’s time to create a key for the seven tree specimens
Now it’s time to create a key for the seven tree specimens. Remember that the key must be dichotomous, and we’ll start by using a spider key format to organize the trees into successively smaller and smaller groups.

21. For example, for the animal group - person, fish, horse, bird,
and snake - a dichotomous spider key might look like this:
Animal Group
Has no legs
Has legs
Has two legs
More than two legs
Has fins
and scales
Doesn’t
have fins
and scales
Horse
No feathers
Fish
Has feathers
Snake
Person
Bird

22. Once you have created the spider key, you must number the choices
- each group of two has the same number, and either an a) or a b),
for example:
Animal Group
1 a)
Has no legs
1 b)
Has legs
2 a)
3 a)
3 b)
2 b)
Has two legs
More than two legs
Has fins
and scales
Doesn’t
have fins
and scales
Horse
4 a)
4 b)
No feathers
Fish
Has feathers
Snake
Person
Bird

23. The first set of choices is always 1a) and b), but notice that you
can number differently from there. Compare the numbering on
this slide to the previous slide. Both are equally correct.
Animal Group
1 a)
Has no legs
1 b)
Has legs
4 a)
2 a)
2 b)
4 b)
Has two legs
More than two legs
Has fins
and scales
Doesn’t
have fins
and scales
Horse
3 a)
3 b)
No feathers
Fish
Has feathers
Snake
Person
Bird

24. … and so on Now that you have created a numbered spider key,
rewrite the key by arranging the choices in a list
format so that they fit nicely one below the other
on the page. The list key puts the numbers in
sequential order, and gives “go to” directions or
identifications after each choice:
1a) Has no legs …………………………..go to 2
1b) Has legs………………………………go to 3
2a) Has fins and scales……………………Fish
2b) Doesn’t have fins and scales………… Snake
3a) Has two legs………………………….go to 4
3b) More than two legs…………………..Horse
… and so on

25. So that’s it!
Your teacher will now give you the
materials to make
a dichotomous key of your own.
Good Luck!

26. Maple
Hickory
Horse chestnut
Oak
Poplar
Eastern redbud
Birch