It is important to have all software be portable between different computers and different operating systems. One of the biggest mistakes is use of full file names with directories. Since the software needs to work on VMS and Unix machines, use the routine CONCAT_DIR, to concatenate a logical directory name and a file name. For example, the command IDL> outfil = concat_dir('$DIR_GEN_OBS', 'obs92_40a.01') returns `$DIR_GEN_OBS/obs92_40a.01' and `$DIR_GEN_OBS:obs92_40a.01' for VMS and Unix machines, respectively. Always use logicals pointing to files in the YS directory area; do not write software that points to personal files since those may not be available at other sites.