Indexed sequential files are based on the sequential file access method. Records are added, updated and deleted the same way in both access methods. However, primary keys are handled differently in indexed sequential files. One B-tree index is maintained in the RMS index file for the primary key fields of the RMSfile. This index is updated any time a record is added, changed, or deleted. The reader is referred to the section on secondary indexes for further details of this indexing method.