HOW ASCII GOT ITS BACKSLASH
Computer History Vignettes

By Bob Bemer

As we've shifted from thousands to hundreds of millions of computer users, much history is lost. Few realize that the backslash character did not exist in much text usage prior to 1961, and in no computer until 1958. A paper by Eric Fischer, submitted to the Annals of the History of Computing in early 2000 (and not yet published), unearthed a backslash on the keyboard of the Teletype Wheatstone Perforator, circa 1937-1945. But this was unknown to data processing people, who were stuck even up to the FORTRAN era (beginning 1955) with the Hollerith punch card code. John Auwaerter, V.P. of Engineering of that same Teletype Corp., and Chairman of X3, certainly never mentioned it.

Here is the story for the record.

GENESIS IN IBM's STRETCH COMPUTER

As an Algol enthusiast I envisioned a backslash for new computer character sets, where before and after pairing with the regular slash (or virgule or slant) could serve for the AND and OR operators of the theoretical Algol character set. I found the opportunity when invited by Dr. Werner Buchholz to do the main design of the 120-character set for the Stretch computer (the IBM 7030). This set, shown on page 12 of [1], shows both curly braces, both square brackets, and the reverse slash -- all of which eventually made their way into 7-bit ASCII.

[2] shows no ESCape character for STRETCH, for I did not think of that until after the character set was built into the hardware. There is one instance of ESCape, in a draft British proposal, for I had by that time alerted Hugh MacGregor Ross.

We may take it as absolute proof of genesis, from the early limited character sets, that IBM's STRETCH was the first computer to use the backslash character, for Reference [2] was actually published a couple of years after the design of that machine. And that uniqueness continued. Reference [2] shows no 6-bit set with the backslash as a working character.

From Reference [3]:

"I had called a joint meeting of IBM, SHARE, and GUIDE, to regularize the IBM 6-bit set to become the standard BCD Interchange Code [76]. Frequency studies of symbol occurrence had been prepared, particularly from ALGOL programs. The meeting of 1961 July 6 produced general agreement on a basic 60-64-character set, which included the two square brackets and the reverse slant, which was chosen in conjunction with "/" to yield 2-character representations for the AND and OR of early ALGOL. This is reflected in the set I proposed to ANSI X3.2 on 1961 September 18."

     (Note: I had put the backslash in position 5/15. It enabled the ALGOL "and" to be "/\" and the "or" to be "\/".)

SHARE and GUIDE representatives at the meeting were a little stubborn about accepting my proposed backslash, so I asked for a character more important to have. After much discussion they could not agree on a better candidate.

"At the 61 November 8-10 meeting, X3.2 constructed the first formal proposal, X3.2\1 ..."     (which, much modified, was to become ASCII)

      (Note: In this proposal the backslash was moved to position 5/12, and there it has remained ever since.)

Of course the need to use pairs of existing symbols to represent a symbol not in the set is long past. So the backslash became that most useful of characters -- one nobody had used or preempted, just waiting there for a new use. The DOS system that Microsoft bought gave us that use in the directory mechanism!

Which is why I refer to the backslash as "my character", together with the other 10 characters I contributed to ASCII -- see this in story.

REFERENCES

  1. R.W.Bemer, W.Buchholz, "An extended character set standard",
    IBM Tech. Pub. TR00.18000.705, 1960 Jan, rev. TR00.721, 1960 Jun
  2. R.W.Bemer, "Survey of coded character representation",
    Commun. ACM 3, No. 12, 639-641, 1960 Dec
  3. R.W.Bemer, "A view of the history of the ISO character code",
    Honeywell Computer J. 6, No. 4, 274-286, 1972
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