Alexander writes: "Each pattern is a three-part rule, which expresses a relation between a certain context, a problem, and a solution."

What is a good design pattern?
A good pattern can be identified because:

• It solves a problem: Patterns capture solutions, not just abstract principles or strategies.
• It is a proven concept: Patterns capture solutions with a track record, not theories or speculation.
• The solution isn't obvious: Many problem-solving techniques try to derive solutions from first principles. The best patterns generate a solution to a problem indirectly--a necessary approach for the most difficult problems of design.
• It describes a relationship: Patterns don't just describe modules, but describe deeper system structures and mechanisms.
• The pattern has a significant human component (minimize human intervention). The best patterns explicitly appeal to aesthetics and utility.