THE question mark is known also as an interrogation mark, occasionally interrogation point, and alternatively as mark or note or point of interrogation. The exclamation mark, occasionally exclamation point, is alternatively mark or note or, rarely, point of interrogation. Both exclamation point and interrogation point are much commoner in the United States than in Britain, where the predominant terms are exclamation mark and question mark; question mark often occurs also in the U.S.A.
Both the question mark (?) and the exclamation mark (!) are supernumeraries or, at best, supplementaries. Although strictly they are rhetorical or elocutionary rather than punctuational, yet, the one consisting of over a . or period, the other of a ! over a . or period, they do normally serve as periods or full stops. In short, they have a double function: rhetorical and punctuational. The of ? represents, I think, a q, short for Latin quaere, imperative of quaerere, to ask, to query *; the ! of !, probably a pointer, perhaps a dagger. Bilderdijk †, however, thinks ? to consist of the q and o of L. quaestio, a question, the former placed over the latter, with the o subsequently diminishing to a dot; and ! to consist of the Latin Io, the I being set over the o and the o diminishing to a dot here too. But it is to be noted that Io is usually written io (from Greek iō); on the other hand, Bilderdijk's theory is supported by the fact that io is an exclamation of joy or triumph.
A few examples will clarify the use of these two marks:
When shall I see you again?-Never!
Why? I asked.-Because it suits me never to see you again, heaven help me!
Hurrah! Hurrah! Hurrah!
* 'Is there not some probability of the question mark's being a simple inversion or reversal of the Greek question mark [;]?': JOHN W.CLARK, in a communication dated 24 November 1952.
† The Dutch writer Willem Bilderdijk (1756-1831) published in 1820-23 a four-volumed work (Taal) in which he dealt most learnedly with questions of poetry and language.