The plurals of nouns ending in -o, owing to the absence of any settled system, are often confusing. The Concise Oxford Dictionary says (p. vi): ‘It may perhaps be laid down that on the one hand words of which the plural is very commonly used, as potato, have almost invariably -oes, and on the other hand words still felt to be foreign or of abnormal form, as soprano, chromo, have almost invariably -os.’ The following is a short list, showing spellings preferred: