* Literal: The cornerstone of the new library was laid by the mayor.
* Figurative: Honesty is the cornerstone of a strong relationship.
* Abstract: Education is a cornerstone of a thriving society.
* Formal: The cornerstone of our business plan is customer satisfaction.
* Informal: Good food is the cornerstone of any good party.
The meaning of "cornerstone" depends on the context. It can refer to a literal stone, or it can be used figuratively to describe something that is essential and foundational.