Files on the S/36 may be Sequential (S), Direct (D), or Indexed (I). An indexed file can have multiple alternate indexes (X), and in fact, a sequential file may have alternate indexes placed on it so there is no primary index. An indexed file contains a key, which must be contiguous and may be up to 60 characters long; however, alternate indexes may have three-part keys which are not contiguous with one another. Duplicate keys in indexed or alternate index files may be allowed or disallowed. A file with direct organization is built with all records added and cannot auto-extend. A file with sequential or indexed organization is built with no records added. An alternate index always has as many records as its parent, as opposed to a System/38-style logical file which is built with conditions to filter records from the parent.