Speakers and learners, based on memory and experience, implicitly know that certain language elements naturally pair together. However, they also understand, through abstract and frequency-independent categories, why some combinations are possible and others are not. The frequency-grammar interface (FGI) bridges these two types of information in human cognition. Due to this interface, the sediment of statistical calculations over the order, distribution, and associations of items (the regularities) and the computation over the abstract principles that allow these items to join together (the rules) are brought together in a speaker's competence, feeding into one another and eventually becoming superposed. In this volume, it is argued that a specific subset of both first and second language grammar (termed 'combinatorial grammar') is both innate and learned. While not derived from language usage, combinatorial grammar is continuously recalibrated by usage throughout a speaker's life. In the domain of combinatorial grammar, both generative and usage-based theories are correct, each shedding light on just one component of the two that are necessary for any language to function: rules and regularities.