The documentation of architectural knowledge helps to understand and prevent the violation of previous decisions. Documenting the architectural knowledge of product lines is more complex than in the case of single-product architectures because it requires considering the existing variability in a product family. This implies having the capability to describe design decisions and relationships related to variability. This paper presents the concept of Product-Line Architectural Knowledge (PLAK): the knowledge of product line architecture and its variability. This concept has been realized through a solution based on models and traceability between models called the PLAK Model.