The combination of a comma and a dash to express apposition is most useful when the thing to be explained by the appositive words is suggested, as in No. 34, and not indicated, as in No. 33. We find a good illustration of this point in Gray’s Elegy as punctuated by the author; and we also find in this illustration what, we believe, is a late development of the dash. The poet Gray was one of the most painstaking writers known in literature. There was a comma in the first line of the manuscript of his Elegy in a Country Churchyard when sent to the printer:
35A. The curfew tolls, the knell of parting day.
The comma here means just what the comma in No. 34 means. What follows “tolls” is in apposition with the thing which “the curfew tolls” suggests, as what follows the comma and dash in No. 34 is in apposition with a picture suggested by what precedes. The suggestion in No. 35A is less apparent and more subtle than in No. 34.
The printer who received the manuscript of the Elegy did not see the picture, and so left out the comma, thus making the intransitive verb “tolls” a transitive verb. The poet’s musical ear felt the improvement made thus unconsciously by the printer; and the change was accepted.
In order more clearly to show the meaning of the manuscript line, we should punctuate it with the comma and dash:
35A-1. The curfew tolls,—the knell of parting day.
Thus the dash is an aid to the comma in grouping the appositive words, especially when following a thought only implied.
There is a very common use of the dash that is commended by all writers on punctuation; but not one of these writers has formulated a rule that differentiates the dash so used from marks of parenthesis.
A writer frequently uses a group of words that are purely parenthetical in nature; and yet he desires to give them grammatical connection with the sentence. Such connection clearly takes them out of the class of groups of words requiring marks of parenthesis; but, because of their purely parenthetical nature, it does not put them into the class requiring commas or semicolons alone. As the group retains a twofold nature, such nature may well be shown by two marks. If it be said this is not good reasoning, we may well resort to a rule which, with the explanation of the conditions above given, together with a very general practice, will quite distinctly differentiate the class of sentences to be thus punctuated. Such a rule will have, at least, the merit of producing a certain degree of uniformity in punctuation. The rule may be about as follows:
Rule.—Matter that is purely parenthetical in nature, and yet is given grammatical connection, slight or otherwise, with what precedes it, may be set off by a dash or dashes together with the mark that would be required by the language with the parenthetical matter omitted.