Engelbart’s NLS system, subsequently renamed ‘Augment’ had a tape backup system that was new to me and so I report it here. The system had run on an SDS 940 but later on a DEC PDP-10. I describe the PDP-10 version. The file directory was modified to include two reel numbers per file. At one time each number denoted a reel of magnetic tape onto which some perhaps previous version of the file had been written. Reels were not reused until they were no longer referred to in any such directory. The tape drives on the PDP-10’s were somewhat unreliable and this provided a backup to the backup. Occasionally special backups were made of directories which might be used upon corruption of the directory on disk.