A B-tree is an index that is made up of two parts: the sequence set and the index set. (These terms are used by IBM’s VSAM file organization documentation. You may encounter other synonymous terms.) The sequence set is an index containing an entry for every record in the file. This index is in physical sequence, usually by primary key value. This arrangement allows sequential access to the data records, as follows: Process the sequence set in order, read the address of each record, and then read the record.