## Definition

A Context-Free Grammar (CFG) is a -tuple

– A finite set of variables.

– A finite set of terminals. is not part of

– A finite set of rules. A rule is of the form

– A single start variable.

## Observations

- cannot be empty. It must contain the start variable .
- can be empty. In that case, all the rules will have on the RHS.
- can be empty. However, this will define the empty language.