/dev/makefile on the V7 distribution tape (or at least the unpacked image I have that I believe to be same) says: ht: /etc/mknod mt0 b 7 64 /etc/mknod mt1 b 7 0 /etc/mknod rmt0 c 15 64 /etc/mknod rmt1 c 15 0 /etc/mknod nrmt0 c 15 192 /etc/mknod nrmt1 c 15 128 chmod go+w mt0 mt1 rmt0 rmt1 nrmt0 nrmt1 According to /usr/sys/dev/ht.c, the minor device number was used as follows: minor(dev) & 07 slave unit number minor(dev) & 070 controller unit number minor(dev) & 0100 tape density: set == 800 bpi, clear 1600 minor(dev) & 0200 no-rewind flag It takes some digging in the source code (and the PDP-11 Peripherals Handbook) to understand all this numerology. In most of the code, minor(dev) & 077 is just treated as a unit number (fair enough). The use of 0200 appears only as a magic number in htopen; that of 0100 only as a magic number in htstart, and that only implied: the test is not minor(dev) & 0100, but unit = minor(bp->b_dev) & 0177; if(unit > 077) Not so bad when the whole driver is only 376 lines of code, but it wouldn't have hurt to make it 400 lines if that meant fewer magic numbers. Anyway, what all this means is that /dev/*mt0 and /dev/*mt1 both actually meant slave 0 on TU16 controller 0, but mt0 was 800 bpi and mt1 1600 bpi. Hence, I would guess, tar's default to mt1. My first exposure to the insides of UNIX was in the High Energy Physics group at Caltech. Some of our systems had multiple tape drives and every drive supported multiple densities, so we invented for ourselves a system like that many other sites invented, with names like /dev/rmt3h to mean the third tape drive at high density. (Hence the USG naming scheme of /dev/rmt/3h and the like--not that we taught it to them, just that many places had the same idea.) Our world wasn't nearly as exciting as that of our neighbors, across the building and three floors down, in the Space Radiation Laboratory. They had a huge room full of racks of magtapes full of data from satellites, and many locally- written tools for extracting the data so researchers could work on it. The hardware was an 11/70 with eight tape drives, and at any given time at least half the the drives would be spinning. One of the drives was seven-track rather than nine-track, because some of the satellite data had been written in that format. Fair disclosure: I had a vague memory that the `drive number' in the device name had been recycled for other purposes, but couldn't remember whether it was density or something else. (I'm a little surprised none of the other old-timers here remembered that, but maybe I worked with tapes more than them.) But I had to dig into the source code for the details; I didn't remember all that. And I did have to climb up to the high shelf in my home office for a Peripherals Handbook to understand the magic numbers being stuffed into registers! Norman Wilson Toronto ON