A cline is a scale of language items that goes from one extreme to another, for example, from positive to negative, or from weak to strong.

Example
The teacher asks learners to map modal verbs of probability such as ‘must', ‘might not' and ‘may' on a cline of probability from 100% sure to 100% not sure.

In the classroom
Clines can be a useful tool for learners to record information, and they can help teachers as a concept check. However, a learner's ability to use a cline does not mean that they can use the target language in context.