When it is longer and more independent, it is generally marked off by dashes, or enclosed in marks of parenthesis. The latter are less frequently used at present than formerly.
The connection of the mail with the state and the executive government—a connection obvious, but yet not strictly defined—gave to the whole mail establishment an official grandeur.—De Quincey.
Note. Brackets are used to indicate insertions that are not part of the text.
III
The clauses of a compound sentence may be separated by colons, semicolons, or commas.
1. The colon is used—
a. To show that the second of two clauses repeats the substance of the first in another form, or defines the first as an appositive defines a noun. Thus,—
This was the practice of the Grecian stage. But Terence made an innovation in the Roman: all his plays have double actions.—Dryden.
b. To separate two groups of clauses one or both of which contain a semicolon. Thus,—
At that time, news such as we had heard might have been long in penetrating so far into the recesses of the mountains; but now, as you know, the approach is easy, and the communication, in summer time, almost hourly: nor is this strange, for travellers after pleasure are become not less active, and more numerous, than those who formerly left their homes for purposes of gain.—Wordsworth.