Cornerstone

A cornerstone is a central part or idea. It is an important element that other parts of something are built around. For example, if you were building a house, you would use cornerstones to make sure that all of the walls of the house were lined up in the right way. Without cornerstones, the walls would not be straight and everything would be uneven. Cornerstones provide structure in the same way that a good foundation provides a strong base for a house.