The CDF library supports two programming interfaces, the Standard Interface and the Internal Interface. The Standard Interface is similar to the interface provided with Version 1 of CDF with several additions for new features. The Internal Interface is provided to allow additional functionality to be added to the CDF library without the need to modify the Standard Interface. Those features not available using the Standard Interface are made available using the Internal Interface (e.g., access to zVariables). The Internal Interface makes CDF extendable. The Standard and Internal interfaces are callable from both C and Fortran.