In certain words, such as _cruel_, metrical custom preserves a
pronunciation in which the adjacent vowels have separate syllabic value.
Traditional grammar, represented by the Academy, asserts that such is
the correct pronunciation of these words to this day; but the actual
speech of the best speakers diphthongizes these vowels, and their
separation in poetry must rank as a dieresis. In printing poetry it is
customary to print the mark of dieresis on many words in which dieresis
is regular as well as on those in which it is exceptional.