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According to a grammar book, dashes are used to link two parts of a sentence or to emphasize by-the-way, parenthetical expressions. I went onto (Should I say, on to???) peruse some good examples and no where did I see expressions formed in dashes with conjuntive words like and and but, but in the posts here in this forum I do and did see some instances of it. Is it acceptable in formal writng?

from a post here; I think from CalifJim:

Sometimes, for the sake of clarity -- and that is left to the judgement of the author -- an extra comma makes sense.
Comments  
A pair of dash can be used to set off an interruptive word, phrase, or dependent clause within a sentence.

I think it does not matter what you put between a pair of dash.
Search The New York Times
http://www.nytimes.com
with
"-- and that"
and you will find:

... Circuit Gilles Villeneuve -- and that has made this week vintage ... appreciate an athlete -- and that's what these guys are ...

... and go somewhere -- and that didn't happen, either. ..

... be almost free -- and that is not what the product ...

... name three others -- and that is what made Agassi's ..
Students: We have free audio pronunciation exercises.
Also:

  1. Saudi Religious Cops Ban Dog, Cat Sales

    ... play with them -- but it isn't a widespread custom ...

    September 8, 2006 - By THE ASSOCIATED PRESS Emotion: travel - World - News
  2. Reporting on British Trials a Challenge

    ... in hefty fines -- but rarely says the restrictions go ...

    September 8, 2006 - By THE ASSOCIATED PRESS Emotion: travel - World - News
  3. Infighting Still Dividing Blair' s Party

    ... buy Blair time -- but not much. He is ...

    September 8, 2006 - By THE ASSOCIATED PRESS Emotion: travel - World - News
  4. Five Years Later: ' Safer but Not Safe'

    ... on highest alert -- but there are, at least ...

    September 8, 2006 - By THE ASSOCIATED PRESS Emotion: travel - News
Parenthetical remarks set off by dashes in informal personal communications (such as posts) and in journalism (especially in what we might call "Sunday Supplement Style") are perfectly acceptable. These are marks of a chatty style. They should not be used in a formal paper such as might be submitted to a scientific journal or to a similar academic organization for publication. They are rarely used in serious front-page stories even in the context of journalism.

CJ
CalifJimParenthetical remarks set off by dashes in informal personal communications (such as posts) and in journalism (especially in what we might call "Sunday Supplement Style") are perfectly acceptable. These are marks of a chatty style. They should not be used in a formal paper such as might be submitted to a scientific journal or to a similar academic organization for publication. They are rarely used in serious front-page stories even in the context of journalism.
Are you talking about the em dash here?
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Em dash

Traditionally typewriters had only a single hyphen glyph so it is
common to use two monospace hyphens strung together-like this-to
serve as an em dash.


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Students: Are you brave enough to let our tutors analyse your pronunciation?
I plead ignorance. I will have to learn more about that term.
Is there an en dash, too? Aren't these two simply the dash and the hyphen?

CJ
Yes, essentially an en dash is a hyphen and an em dash is a 'dash'. The en/em terms are just old typesetter terms to distinguish between the two and it is not really necessary to know about them.

— em dash

– en dash

although I notice that Microsoft still calls them emdash and endash rather than hyphen and dash if you go to 'insert symbol'. It actually takes an effort to use a proper em dash, most people just use an en dash or two en dashes next to each other.
Ugh. I have to use em dashes and en dashes so often I know the ascii code for them.

En is used to connect things like a date range: Nov. 17-19 (this would use an en-dash, but on this computer I can't make one).

However, 98% of the people who read what you write won't care if it's an en dash or not, and of the remaining 2%, I work with a dozen of them, so you're pretty safe using a hyphen.
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