Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
PLURALS OF NOUNS ENDING IN -O
sometimes retain a foreign plural (pizzicato becomes pizzicati), sometimes form their plurals by adding an -s (piano becomes pianos), sometimes by adding -es (potato becomes potatoes; tomato becomes tomatoes), and sometimes they may have two plurals (contralto becomes contraltos or contralti; salvo becomes salvos or salvoes).
