DeVinne remarks upon this use of the colon that it is traditional and can not be explained.
The colon is sometimes used between the hours and minutes in indicating time, like: 11:42 a.m.
DeVinne does not approve of this, though other authorities give it as the rule. It is probably better to use the period in spite of its use as a decimal point, which use was probably the motive for seeking something else to use in writing time indications. In railroad printing the hour is often separated from the minutes by a simple s.p.a.ce without any punctuation.
THE PERIOD
The period, or full stop, marks the end of a declarative sentence. As a sign it has several other uses which will appear in the paragraphs following.
_Rules for the Use of the Period_
1. At the end of every sentence unless interrogative or exclamatory.
2. After abbreviations.
Nicknames, _Sam_, _Tom_, etc., are not regarded as abbreviations.
The metric symbols are treated as abbreviations but the chemical symbols are not. M. (metre) and mg. (milligram) but H_{2} O and Na Cl.
Per cent is not regarded as an abbreviation.
The names of book sizes (12mo 16mo) are not regarded as abbreviations.
The period is now generally omitted in display matter after
Running heads, Cut-in side-notes, Central head-lines, Box heads in tables, Signatures at the end of letters.
The period is omitted
After Roman numerals, even though they have the value of ordinals.
After MS and similar symbols.
In technical matter, after the recognized abbreviations for linguistic epochs. IE (Indo-European), MHG (Middle High German)
and after t.i.tles of well-known publications indicated by initials such as AAAPS (Annals of the American Academy of Political Science).
When a parenthesis forms the end of a declarative sentence the period is placed outside the parenthesis, as in the preceding example. A period is placed inside a parenthesis only in two cases.
1. After an abbreviation.
This was 50 years ago (i.e. 1860 A.D.)
2. At the end of an independent sentence lying entirely within the parenthesis.
Lincoln was at the height of his powers in 1860 (He was elected to the presidency at this time.)
When a sentence ends with a quotation, the period always goes inside the quotation marks.
I have just read DeVinne"s "Practice of Typography."
The same rule applies to the use of the other low marks, comma, semicolon, and colon, in connection with quotation marks. Unlike most rules of grammar and punctuation, this rule does not rest on a logical basis. It rests on purely typographic considerations, as the arrangement of points indicated by the rule gives a better looking line than can be secured by any other arrangement.
_Other Uses of the Period_
1. The period is used as a decimal point.
2. The period is used in groups, separated by s.p.a.ces, to indicate an ellipsis.
He read as follows: "The gentleman said . . . .
he was there and saw . . . . the act in question."
THE DASH
The dash is a very useful mark which has been greatly overworked by careless writers. It is very easy to make in ma.n.u.script and serves as a convenient cover for the writer"s ignorance of what point should properly be used.
The conspicuousness of the dash makes it a very useful mark for guiding the eye of the reader to the unity of the sentence. It is particularly useful in legal pleadings where there is much repet.i.tion of statement and great elaboration of detail. In such cases commas, semicolons, and even parentheses are so multiplied that the relation of the clauses is lost sight of. The confusion thus arising may often be cleared up by intelligent use of the dash.
The dash is sometimes used to connect a side heading with the text that follows, or to connect the end of that text with the name of the writer.
A RULE FOR PEACE.--If it be possible, as much as lieth in you, live peaceably with all men.--_St. Paul._
The dash is sometimes used in catalogue work as a ditto mark.
DE VINNE, THEODORE LOW. Historic Printing Types. New York, 1886.
----The Invention of Printing. Francis Hart & Co., New York, 1878.
----Plain Printing Types. Oswald Publishing Co., New York, 1914.
French printers use the dash in printing dialogue as a partial subst.i.tute for quotation marks. Quotation marks are placed at the beginning and end of the dialogue and a dash precedes each speech. This form is used even if the dialogue is extended over many pages.
_Rules for the Use of the Dash_
1. To mark abrupt changes in sentiment and in construction.
Have you ever heard--but how should you hear?
2. To mark pauses and repet.i.tions used for dramatic or rhetorical effect.