Intelligent network capability set 1 is the first standardized stage of the intelligent network (IN) as an architectural concept for the creation and provision of telecommunications services. This Recommendation gives an introduction to capability set 1 (CS-1) by providing an overview and definition of CS-1 and by describing its main characteristics and overall capabilities. It defines the service aspects, network aspects and functional relationships that form the basis of the CS-1 capabilities. This Recommendation is the first in the Q.121x-Series Recommendations devoted to CS-1. It builds on the architectural principles of IN as described in the Q.120x-Series Recommendations. The CS-1 Recommendations (Q.121x-Series) form a useful basis for achieving implementation experience. As with any project of this size and complexity, it can be anticipated that there may be some difficulties in interworking the various implementations of IN CS-1 physical elements. In order that the IN objective for working in a multi-vendor environment may be fully achieved, the IN CS-1 Recommendations text may go through some future revision in the light of implementation experience