If there is no comma where the clause is to be inserted, dashes alone should be used:
“In the completed volume of the third report, the countries wherein education has received the most attention are treated of at length.”
If a parenthetic clause be inserted after “countries,”—where there is no comma,—only dashes are required; thus:
“In the completed volume of the third report, the countries—Prussia, for instance—wherein education has received the most attention are treated of at length.”
A thin space should be placed before, and also after, a dash.
If a parenthesis is inserted in a part of a sentence {p92} where no point is required, no point should be placed before or after the marks of parenthesis.
“By living sparingly, and according to the dictates of reason, in less than a year I found myself (some persons, perhaps, will not believe it) entirely freed from all my complaints.”—Cornaro.
As a general rule, if the parenthesis occur after a punctuated clause, the point should be placed after the latter mark of parenthesis.
“Popham’s monument, by the intercession of his wife’s friends (who had interest at Court), was left in St. John’s Chapel on condition either of erasing the inscription, or turning it inwards.”
“Artist: Kneller (1723). Architects: Taylor (1788); Chambers (1796); Wyatt (1813).”