THE DASH-written singly or, as in this interpolation, in pairs-resembles parentheses, in that it is supernumerary to the four true marks of punctuation-period, comma, colon, semicolon. The dash further resembles parentheses, in that, in one important function, it expresses rather more strongly, rather more abruptly, what parentheses express less strongly and much more smoothly.
A dash derives from 'to dash', to shatter, strike violently, to throw suddenly or violently, hence to throw carelessly in or on, hence to write carelessly or suddenly, to add or insert suddenly or carelessly to or in the page. 'To dash' comes from Middle English daschen, itself probably from Scandinavian-compare Danish daske, to beat, to strike. Ultimately the word is-rather obviously-echoic.
The employment of the dash falls into two main divisions: strictly punctuational; in the main, non-punctuational. There is, by the way, no fundamental difference between a single dash and a pair of dashes: usually position rather than function is the determining factor.
A pair of dashes, sometimes a single dash in the final position, can serve the same purpose as a pair of parentheses-but in a slightly different way. The difference consists in degree. The dash or the pair of dashes sets off the parenthetical matter both more clearly and more decisively: when parenthetical, the dash or dashes are stronger. Thus:
John the Hedger-to be accurate, he was a blacksmith-acted so eccentrically that people began to wonder-very naturally wonder-whether he were not, in fact, mad.
(I should have punctuated: John…people began to wonder, very naturally wonder, whether…)
The country's exports might have gravely-but certainly not disastrously-decreased in that short period.