The DRY principle:

Don’t Repeat Yourself (also known as Once and Only Once or as Single Point of Truth)  is a phylosophic process used in IT  that aims to reduce the duplication of the information.

Duplication cons:

The principle emphasized by this theory is that the informations should not be duplicated because the duplication increases the difficulty to change things and decreases clarity bringing often situations of inconsistency.

The DRY creators:

DRY is one of the principles described in the book The Pragmatic Programmer written by Andy Hunt and Dave Thomas .

What can be applied to?

This theory can be applied to virtually everything; for example can be applied to database schemas, to test strategies, to the building of systems and even the documentation.

Which are the DRY approach pros?

When the DRY principle is applied properly, the modification of an an element does not change other elements that are logically unrelated to It. The elements that are logically related to it will all change in a predictable and uniform way so they are therefore kept synchronized.