+537.+ In our study of the comma and the dash we have found that
parenthetical statements are set off from the rest of the sentence
sometimes by a comma and sometimes by a dash. When the connection with
the rest of the sentence is close, and yet the words are thrown in in a
parenthetical way, commas are used to separate the parenthetical
statement from the rest of the sentence.
+538.+ When the connection is not quite so close, the dash is used
instead of the comma to indicate the fact that this statement is thrown
in by way of explanation or additional statement. But when we use
explanatory words or parenthetical statements that have little or no
connection with the rest of the sentence, these phrases or clauses are
separated from the rest of the sentences by the parenthesis.
+539.+ +GENERAL RULE:--Marks of parenthesis are used to set off
expressions that have no vital connection with the rest of the
sentence.+ For example:
Ignorance (and why should we hesitate to acknowledge it?) keeps us
enslaved.
Education (and this is a point that needs continual emphasis) is the
foundation of all progress.