2. NOUNS ENDING IN "O." If the final "o" is preceded by a vowel, the
plural is formed regularly, i.e., by adding "s": as, cameo, cameos. If
the final "o" is preceded by a consonant, the tendency of modern usage
is to form the plural by adding "es": as, hero, heroes; potato,
potatoes. The following common words, however, seem still to form the
plural by adding "s" alone:--

canto lasso proviso torso
duodecimo memento quarto tyro
halo octavo solo
junto piano stiletto

3. NOUNS ENDING IN "Y." If the "y" is preceded by a vowel, the plural
is regular: as, valley, valleys.

If the "y" is preceded by a consonant, "y" is changed to "i" and "es" is
added to form the plural: as, lady, ladies; city, cities.

4. PROPER NOUNS are changed as little as possible: as, Henry, Henrys;
Mary, Marys; Cicero, Ciceros; Nero, Neros.
5. Most COMPOUND NOUNS form the plural by adding the proper sign of the
plural to the fundamental part of the word, i.e., to the part which
is described by the rest of the phrase: as, ox-cart, ox-carts;
court-martial, courts-martial; aide-de-camp, aides-de-camp.

Note the difference between the _plural_ and the _possessive_ of compound
nouns,--forms which are often confounded. See page 16.

6. Letters, figures, and other symbols are made plural by adding an
apostrophe and "s" ('s): as, "There are more _e's_ than _a's_