The phrase "forward slash" contains a space, while its equivalent "backslash" does not. This seems inconsistent; should "forwardslash" not be a valid word?

From Wikipedia I discovered that slash, rather than being a group term, is actually the correct name for a forward slash; with the forward qualifier simply being added for clarity.

Is there a rule underlying the use of the space in "forward slash" but not in "backslash", or is it just down to typical usage?

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    Comments are not for extended discussion; this enlightening and cheeky conversation has been moved to chat.
    – Kit Z. Fox
    Dec 2, 2015 at 18:49

4 Answers 4


Forward slashes have been around forever (well, at least since Ancient Rome), but they were just called "slash".

Backslashes, on the other hand, are a fairly recent invention. How ASCII Got its Backslash reports that it was added to the ASCII character set in 1961 for the Algol programming language. The term rose in popularity in the early 80's, probably tied to the rise of MS-DOS.

Why is it "backslash" and not "backward slash"? Simply because that's what the guy who invented it(*) called it and it stuck. Had he called it "backward slash", it's debatable whether common use would have eventually shortened it to "backslash".

"Forward slash" is used comparatively less often, and only when someone wants to be really specific about which slash to use. I still hear it called just "slash" a lot.

As for the "rule" that governs whether it's written as one word or two? Compound words in English can be written with or without a hyphen or space; it just comes down to common usage. As this article points out:

Over time, the convention for writing compounds can change, usually in the direction from separate words (e.g. clock work), to hyphenated words (clock-work), to one word with no break (clockwork).

(*) Technically the backslash existed in some form prior to its inclusion in ASCII, but it was not in widespread use and I could find no evidence of its name prior to that point.

  • @ruakh - Well, yes, there's no law that says we have to preserve the inventor's name. All I meant was that he called it that and it stuck. Had he called it "backward slash" instead, it's debatable whether people would have still eventually shortened it to "backslash" or kept the original name.
    – Lynn
    Nov 25, 2015 at 17:10
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    The Unicode name of "reverse solidus" appears in an academic paper of 1962.
    – Andrew Leach
    Nov 25, 2015 at 19:08
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    If you're looking for older use of the backslash, it is used as the symbol for the set-theoretic difference in mathematics. Most forms of mathematical notation are fairly young, so this notation might only date back to the beginnings of modern set theory around 1870-1900. In this context it is usually pronounced "minus" or something like that. Nov 25, 2015 at 23:48
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    The crucial point, missing from this answer, is that the equivalent to backslash would be foreslash. Cf. foreskin, forehand, foresight, or indeed the very word forward. So writing forwardslash is akin to writing forwardskin, forwardhand, forwardsight. You don't do that. In short: each of backslash and forward slash does follow a clear and regular pattern, it's just that the OP failed to notice they are two different regular patterns. As to why foreslash doesn't exist, there are several reasons, including a) blocking, and b) fore being antiquated and no longer productive.
    – RegDwigнt
    Nov 26, 2015 at 18:54
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    I don’t think they called it slash in Ancient Rome. ;-) Nov 26, 2015 at 20:09

In the unix world it was just called a slash. The backslash was the escape character. When MS-DOS came around and used the wrong slash to delimit subdirectories, programmers referred to that as slash also; the context determined whether it was one or the other. It was not until non-technical people started using URLs that technical writers felt the need to say "forward slash" to keep them from introducing escapes into web addresses.

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    I don't think that a particular OS's conventions being different to what came before it makes that convention wrong. Microsoft didn't re-implement Unix, they designed their own OS. Maybe it was a bad design choice, but there is no inherent right or wrong in it.
    – JBentley
    Nov 25, 2015 at 19:36
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    During my computer science days (1980s), the backslash was called a "slosh" for the purposes of precise differentiation. However it must have been fairly localised to Australia, or even within the one university, as I can't find any references to it.
    – Cargill
    Nov 25, 2015 at 20:20
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    DOS didn't use "the wrong slash". It ended up using a backslash as a path separator for compatibility with CP/M and because options were specified with forward slashes.
    – isanae
    Nov 26, 2015 at 0:13
  • When I was a computing kid, many, many moons ago, in a world before CP/M, the reverse solidus was called hack.
    – bye
    Nov 26, 2015 at 10:45
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    FWIW When I said MS used the wrong slash I was joking.
    – KCCole
    Apr 6, 2017 at 19:27

There is a well expression often used to illustrate the difference between compound words and modifier-modified combinations. You've probably heard "blackboard", the compound, compared with "black board", which is a combination of an adjective modifier "black" and the noun it modifies, "board". Not only is the spelling different, a dividing space being used after the adjective, but the pronunciation is different as well. The compound has more stress on the first part, "black-", while the modifier phrase has more stress on the second part.

The pair "backslash"/"forward slash" are like the above well known pair in both respects I mentioned: only the first is written without a space, and the pronunciation differs, with most stress on the first part of "backslash" but most stress on the second part of "forward slash". I conclude that the grammatical difference is the same: "backslash" is a single word, while "forward slash" is a phrase -- a syntactic combination of two words.

I'm not saying, nor denying, that the "forward" in this phrase is an adjective, like the two model cases I mentioned. I don't know what it is. But I do think that "forward" is an independent word, unlike the "back" in "backslash".

So I think that the conventional spelling of "backslash" without an internal space, unlike "forward slash", makes perfect sense.

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    The White House (in Washington, DC) is written as two words. Nov 25, 2015 at 18:35
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    The classic example that the White House is used for illustrates the use of emphasis. A house that is white is a white HOUSE. The president's place is the WHITE house. Nov 25, 2015 at 19:01
  • @StevenLittman, thanks, so it is. I removed the example.
    – Greg Lee
    Nov 25, 2015 at 19:01

In a nutshell: expertsexchange

Not a very erudite answer here, but common sense would seem to indicate that forwardslash is ambiguous between forward slash and forwards lash.


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