All file systems communicate with the ACUCOBOL-GT generic file handler via interface routines. The interfaces to external file systems such as C-ISAM and the various RDBMSs are available from Acucorp as add-on modules. The user is free to add any combination of traditional ISAM interfaces and RDBMS interfaces that will meet a site's particular needs. The ACUCOBOL-GT runtime system communicates with relational databases via a special family of add-on products called Acu4GL.