A Master Patient Index (MPI), sometimes called a Client Registry (CR), is a foundational piece of any regional or national health system. Fundamentally, a MPI solution seeks to create a single master list of patients across a jurisdiction and to provide a cross-reference of the various identifiers used by local systems in a region to identify a patient. For example, if a patient is registered in Hospital A as patient number 123 and is registered in Primary Clinic B as patient number 456, a Master Patient Index configured in the region will keep track of these various identifiers and provide a list of them to any system on demand. This enables health care providers to see a "longitudinal view" of a patient, that is, all interactions of the patient with various components of the health care system over time.