Understanding the OSI Model
THe OSI model is very idealistic and deals with the hierarchical relationship between layers in the protocol stack. The interdependencies may be uniquely defined for any application, as they will differ from each other as it might on any operating system.  The  7 layers were shown visually in a pyramid to define that each layer was dependant on all the layers below it as dependants and all the layers above layers as precedents.

These interdependencies for each application when understood, become a knowledgebase for cause and effect. This relationship can often be measured statistically for correlation with overall performance or use by ITIL support staff for troubleshooting. Isolating the root causes of a problem requires symptoms with previous experimental results and correlation to present symptoms and experiments.

Q  Do these layers build on one another, or are some of them completely parallel?

A Yes. For most applications they lay the foundation for each layer, but in some cases may run as infrastructure layers to support the above layer.