I have seen some writing where people have a list or a figure in writing and they will write something like this:
The information is provided in Image 3:-
Is that correct? Is this a British style?
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According to Nick Marten's The Secret History of Typography in the Oxford English Dictionary, a colon followed by a dash is a typographical mark that the OED refers to as the dog's bollocks:
Citing usage from 1949, the OED calls this mark the dog’s bollocks, which it defines as, “typogr. a colon followed by a dash, regarded as forming a shape resembling the male sexual organs.” This is why I love scrounging around the linguistic scrap heap that is the OED. I always come across a little gold. And by “gold,” I mean, “vulgar, 60-year-old emoticons.”
Marten does not further elaborate on its purported usage, but others do:
In Britain the exclamation mark is sometimes referred to as a dog’s prick, and that, further, the combination of a colon and a dash (:—), out of fashion now but long used to represent a restful pause, is known as a dog’s bollocks.
Modern style guides seem intent on banishing its usage to history. For example, the University of Sussex has a strong opinion on the matter:
The colon [is] never preceded by a white space; it is always followed by a single white space in normal use, and it is never, never, never followed by a hyphen or a dash — in spite of what you might have been taught in school.
I'd love to find some examples in print, but as you can imagine:— it's extraordinarily difficult to google.
English documents written in India often use
:-. For example:
Note :- [PDF]
(These quotes are merely illustrative and not meant to prove that
:- is commonly used in India; however, I have read a number of English documents written in India and can assure you that
:- is far more prevalent there than in the US.)
I do not know why
:- is common in India (British influence, maybe?), but there you have it. My impression is that
:- is used to introduce a list, and
: is used in all other circumstances. If somebody could get their hands on a style guide used by the Indian government (if such a thing even exists ― it probably doesn't), that would probably shed some light on this.
The interested reader who seeks additional examples may wish to consult this Stack Exchange Data Explorer query, which identifies instances of
:- on Hinduism.SE (which is, of course, largely populated by Indian users).
Looking around this page, colon + hyphen appears to be common to a number of Commonwealth and British territories, though there are exceptions to that rule (e.g. Phillipines).
Usage in legislation would seem to indicate that it is indeed "proper" to use punctuation in this way. The fact that its use is mainly (though not exclusively) in countries with a British connection suggests that it may be more a British style than a non-British one.
From the Constitution of the United States of America
Article II, Section 1, last clause
Before he enter on the Execution of his Office, he shall take the following
Oath or Affirmation:—‘‘I do solemnly swear ...
This version is from the 110th Congress, dated July, 2007.