Double underlining in letter-writing need not be here adverted to. If the person to whom one writes a letter is likely to read it without appreciation or care, one is entitled to adopt any means that will ensure attention. But if double underlining is allowable only on this ground, general rules are obviously of no use.
LXIV. Words from a foreign language which have not become classical English words, are written in italics.
The slightest _double entendre_ made him blush to the eyes.
When foreign words become English, they are no longer italicized. Among such words are: rationale, aide-de-camp, quartette, naïve, libretto. It is often a matter of discretion to say whether a word is so far naturalized that it should be written in the ordinary way.
LXV. Names of newspapers and magazines, and names of ships, are generally written in italics; as the _Times_, the _Fort-nightly Review_, the _Great Eastern_.
LXVI. The hyphen is used between the component parts of some compound words.
Paper-knife; book-keeping; coal-pit; water-carrier; printing-press; sea-water; man-of-war; now-a-days; high-art decoration; good-looking.
There is no rule to distinguish the compound words that take a hyphen from those that do not. If one be in doubt about a particular word, the best thing to do is to refer to a dictionary.
LXVII. When one syllable of a word ends with a vowel, and the next syllable begins with the same vowel, the hyphen is placed between the syllables to indicate that the two vowels do not form a diphthong, that is, that they should not be pronounced together.
Co-operative; co-ordinate; pre-eminently; re-establish; re-echo.
In the same way the hyphen sometimes ensures that two consonants shall be pronounced separately; as in "book-keeping," "shell-less," "cock-crow," "sword-dance."
LXVIII. As a rule, a hyphen should not be placed after a simple prefix: "contravene," "preternatural," "hypercritical," "bilateral."
(_a_) "Anti-religious," "ultra-liberal," "semi-lunar," "co-eval." In these words the pronunciation is more clearly marked by inserting the hyphen. Compare "antiseptic," "antinomian," "ultramontane," "semicircle."
Perhaps among these exceptions should also be included such words as "pseudo-critic," "non-ego," "non-existent." Compare "pseudonym," where the prefix is contracted, and "nonentity." Words like "pre-eminent," divided for the same reason, have already been noted.