CLLI codes are useful to telecommunications companies for ordering phone service, for the rating of call detail records for billing purposes, and to assist in tracing calls. CLLI codes are associated with Vertical and Horizontal coordinates (frequently abbreviated to "V and H coordinates"), which were developed by AT&T researcher Jay K. Donald to provide a relatively simple method of calculating distance between two network locations. Various mileage-sensitive services are priced according to the V and H coordinates associated with the two endpoints' CLLI codes.