KEY
Keys help in classification
Keys are part of classification because they help place an organism into the correct group (like mammals, birds, insects, etc.).
Each step must have ONLY two choices
This is very important for Grade 9:
Every step has two statements
They must be opposites.
Example:
Has leaves
Does not have leaves
There is never a third option.
Keys use observable features
Features must be things you can see quickly, not things you must measure or test.
Examples commonly used in Grade 9:
Body covering
Type of limbs
Flower present or absent
Number of segments
Shape of leaf
Keys start with the easiest difference
The first step always uses the biggest and simplest difference so the group splits quickly.
Example:
Has wings / No wingsis easier than
Long wings / Short wings
Keys become more detailed as you go down
Beginning steps = big differencesLater steps = small differences
This helps narrow down the organism correctly.
Keys must lead to ONLY one organism
A key is wrong if:
two organisms fit the same final step
one organism fits more than one answer
a choice is confusing
Each pathway should end with one unique organism.
Keys can be written in two formats
Grade 9 students should know both:
(a) Numbered steps format
1a. Has wings → go to 21b. No wings → go to 3
(b) Branching key (like a tree)
Has wings → birdNo wings → snake
You must know both forms for exams.
Keys don’t show relationships
A common exam point:
Keys identify organisms
They do NOT show evolutionary relationshipsThat is done by classification, not keys.
Keys help in fieldwork
In Cambridge biology, keys are used in:
field studies
sampling
identifying plants or insects collected during quadrat or transect studies
So they are practical tools, not only theory.
Keys work best with familiar organisms
Grade 9 exam questions usually use:
insects
plants
small animalssince their features are easy to compare.