All peripheral devices allow files to be processed sequentially: you start at the beginning of the file and work through each record in turn. One important advantage of sequential files is that different records can have different lengths; the minimum record length is zero but the maximum is system-dependent.
Sequential files behave as if there were a pointer attached to the file which always indicates the next record to be transferred. On devices such as terminals and printers you can only read or write in strict sequential order, but when a file is stored on disc or tape it is possible to use the REWIND statement to reset this pointer to the start of the file, allowing it to be read in again or re-written. On suitable files the BACKSPACE statement can be used to move the pointer back by one record so that the last record can be read again or over-written.
One unfortunate omission from the Fortran Standard is that the position of the record pointer is not defined when an existing sequential file is opened. Most Fortran systems behave sensibly and make sure that they start at the beginning of the file, but there are a few rogue systems around which make it advisable, in portable software, to use REWIND after the OPEN statement. Another problem is how append new records to an existing sequential file. Some systems provide (as an extension) an ``append" option in the OPEN statement, but the best method using Standard Fortran is to open the file and read records one at a time until the end-of-file condition is encountered; then use BACKSPACE to move the pointer back and clear the end-of-file condition. New records can then be added in the usual way.
The alternative access method is direct-access which allows records to be read and written in any order. Most systems only permit this for files stored on random-access devices such as discs; it is sometimes also permitted on tapes. All records in a direct-access file must be the same length so that the system can compute the location of a record from its record number. The record length has to be chosen when the file is created and (on most systems) is then fixed for the life of the file. In Fortran, direct-access records are numbered from one upwards; each READ or WRITE statement specifies the record number at which the transfer starts.
Records may be written to a direct-access file in any order. Any record can be read provided that it exists, i.e. it has been written at some time since the file was created. Once a record has been written there is no way of deleting it, but its contents can be updated, i.e. replaced, at any time.
A few primitive operating systems require the maximum length of a direct-access file to be specified when the file is created; this is not necessary in systems which comply fully with the Fortran Standard.