PARENTHESES-INDICATED by ()-are primarily punctuational, secondarily a non-punctuational convenience or convention. Parentheses and dashes (Chapter 8) are supernumeraries, the basic stops being the period, the comma, the colon, the semicolon.
Etymologically, parentheses is the plural of parenthesis (with adjective parenthetic or parenthetical), adopted, through Medieval Latin, from Greek. The Greek parenthesis consists of para, beside+en, in+thesis, a placing, from tithenai, to place, put, set: literally, therefore, parenthesis signifies 'an insertion beside'; here, beside the basic meaning of the sentence.
The essence of all parentheses is that, without them, the sentence is grammatically and logically complete: they explain or modify, but they do not determine the sense.
'The test of a parenthesis is whether the other words make sense without it' (CHARLES C.BOYD, Grammar for Great and Small, 1928): if they don't, either the whole or a part of the parenthesis should be removed from within parentheses, as in:
An adjectival clause (which plays essentially the same role as the simplest adjective) may be compared with an adverbial clause, which is nothing but an adverb expressed finitely at some length: where clearly the parentheses should be displaced by commas.
Thus, 'Mr Jones (a famous surgeon) was hastily summoned to perform an urgent operation' remains, in essence, unchanged by the omission of '(a famous surgeon)'. Compare 'Mr Jones (who was a famous surgeon) was hastily summoned…'
The four principal functions of parentheses are: