A key is used to identify the number of categories present in a graph. It is also called a legend

A key on a pictograph tells us how many each picture stands for. Look at the following pictograph

Video Examples: Key Stage 1 Maths: Addition

Example of Key

In the graph shown, the key is given to the right of the graph to notify us which line graph corresponds to which hitter.
The key here is important, because otherwise we wouldn't be able to answer any question comparing the two hitter's performance.

Solved Example on Key

Ques: Find the number of keys in the graph.

Choices:

A. 1
B. 2
C. 3
D. 4
Correct Answer: B

Solution:

Step 1: From the graph, it is clear that there are two categories boys and girls
Step 2: So, the number of keys = 2