Because relational databases handle I/O at the column (field) level, and COBOL handles I/O at the record level, some mapping is necessary to associate records with their fields. One function of the Acu4GL product is to map COBOL records into database fields, and to map the database fields back into records. Acu4GL does this by consulting data dictionaries generated by the compiler. The detailed structure of data dictionaries is discussed in the XFD Files section. The next section shows how and when these data dictionaries are built.