Punctuation is the art of marking the divisions of a sentence by means of conventional signs—the full stop or period (.), colon (:), semicolon (;), comma (,), dash (—), mark of exclamation (!), mark of interrogation (?), inverted commas (""), and brackets—( ), [ ]. Broadly speaking, there are two principal systems of punctuation, the grammatical and the logical. The system most frequently followed in British printing-houses is neither of these, being a set of empirical rules, in which the logical element is almost entirely wanting, the grammatical is present to some extent, but the ruling factor is apparently arbitrary fancy. Commas are too often held in profound contempt, being scattered at random amongst the words as if from a pepper-box. These lawless little adjuncts can be found, in the best-printed books, insinuating themselves between subject and verb in even short sentences. The printing-offices of the United States are to some extent uniform in their practice. The system they follow is much better than those in
Britain, and is based principally upon grammatical laws. The old-fashioned method of putting in a comma (or even a stronger stop) wherever a reader would naturally pause to take breath when reading aloud (as at this point of this sentence) has little in reason to commend it. Punctuation is confessedly difficult, partly owing to the vast differences in the style of different writers, and partly owing to the conflicts between logical meaning and grammatical word-arrangement which in some cases are inevitable. Given a sensible system, practical experience is the best teacher. In theory little more can be done than to lay down a few general maxims for guidance.
In the first place, follow a logical method of sentence subdivision: let the first and foremost aim be to bring out the meaning clearly and unambiguously, in so far as this can be done with the help of stops. Use commas and semicolons sparingly, especially commas; use them, indeed, only where they are absolutely necessary. The sentence should stand on its own feet, not rest upon a long array of comma crutches. It is not as a general rule necessary to set commas to fence or adorn every adverbial clause. Especial care is required in punctuating sentences that contain relative clauses. If the relative sentence is entirely subordinate to the main sentence, or if it gives additional information, separate it by a comma or commas; if on the other hand it belongs essentially to the structure of the thought expressed by the main sentence, put no comma. For example, in 'the man who had an impediment in his speech,' written without the comma, the relative clause points out this particular man and distinguishes him from some other or others who have been also spoken of: it fulfils in fact the function of an article or demonstrative pronoun. In the same sentence printed with the comma, 'the man, who had' &c., the relative clause gives an entirely new piece of information, and is no longer demonstrative. The colon is generally put before a long quotation. It would be well to confine the use of it to this and to one other case—namely, to part a general statement from the immediately following particular application of it or exemplification of it in detail. As for dashes, it is difficult to summarise the rules for their use. They are commonly employed to indicate a sudden break or change in the grammatical structure or the logical development of the sentence, as well as to put ironical emphasis upon a word or words thrown to the end of the sentence, as in Heine's phrase, 'Göttingen is noted for its professors and its—sausages;' but even this would be better without such a clumsy advertisement of the humour. A dash may precede an enumeration of mere names or dates or objects expressed in very brief terms. One dash may also be put before and one after a short clause that merely explains in other words or makes clearer a statement that has just been made; both dashes, and not one dash and some other stop, should be used, except where the second would fall at the end of a sentence. But for this purpose, especially where the parenthetical nature of the added explanatory clause is more prominent, brackets are frequently employed. Semicolons are most appropriately used in compound sentences or sentences that embrace antithetical statements. In the former class of sentence they should mark off the subordinate sentences from the main sentence or co-ordinate sentences from one another; in the latter class they should separate the antithetical sentences, which generally begin with 'but' or some equivalent. The mark of exclamation has another besides its legitimate use: it is frequently put after absurd or highly improbable statements. The mark of interrogation too has a secondary use: placed in brackets immediately after a word it throws doubt upon its correctness, either as according with fact or as being philologically or grammatically correct.
Of course these are only general rules. Many exceptions even to them must necessarily occur. The golden rules in all cases of doubt are two: (1) let logic or, better, common sense be the supreme guide; (2) punctuate so as to bring out the sense best. It is greatly to be desired that British printing-houses would come to some agreement as to a uniform and systematic method of punctuation. See H. Beadnell's Spelling and Punctuation (4th ed. 1891).