In order to retain compatibility with Fortran66, many systems provide other pre-connected files. It used to be customary to have unit 5 connected to the card-reader, and unit 6 to the line printer. Other units were usually connected to disc files with appropriate names: thus unit 39 might be connected to a file called FTN039.DAT or even TAPE39. These pre-connections are completely obsolete and should be ignored: an OPEN statement can supersede a pre-connection on any numbered unit. Unfortunately these obsolete pre-connections can have unexpected side effects. If you forget to open an output file you may find that your program will run without error but that the results will be hidden on a file with one of these special names.