[Buschmann 1996] describes Layers from a more general architectural perspective.
Some concrete layer implementations are instances of Adapter [Gamma 1995] since they adapt their own layer's abstraction to the abstraction for the subsequent layer.
You can define more than one layer using the same abstraction. This comes in handy if the layers implement different features for the same logical database operations. Stacking multiple layers that implement the same abstraction forms an instance of a Chain of Responsibility [Gamma 1995]. Since the layers implement the same abstraction, you can combine them in different orders.
Instances of Data Accessor (9), Active Domain Object (33), Object/Relational Map (53), and Domain Object Assembler (227) can make layers within a single system. It is also convenient to encapsulate resource and cache patterns within one or more layers.