Dichotomous Keys

Keys are a series of choices between two mutually exclusive and parallel statements.

Followed correctly, keys will lead you to the correct name of an unknown object.



Paired, mutually exclusive statements.

The pair of statements is referred to as a couplet.

Each 1/2 of a couplet is a lead.

Divides objects into smaller subsets until only one remains



Leaves green throughout, flowers blue.

Leaves variegated, flowers white.


Key Construction

Subject first, adjectives following, verbs omitted

Numbered, indented, or sometimes both

Statements should be positive


Key Construction

Sizes should be in measurements, not relative

Leads should be parallel


Key Construction--examples

Not parallel:

Plants villous, corollas red.

Leaves glabrous, flowers green.



Leaves villous, corollas red.

Leaves glabrous, corollas green.


Tips for Using Keys

Read both choices

Understand the terms

Observe carefully

Try to study several specimens

If unclear try both legs

Check your conclusions