Titles of longer written works are underlined or italicized.
Longer written works include books, full-length plays, films, longer musical compositions, and periodicals.
Incorrect (speaking of the musical): I like Oklahoma.
Incorrect: I like "Oklahoma."
Correct: I like Oklahoma. OR
I like Oklahoma.
(The title of a longer work is italicized or underlined.)
Correct: I liked Macbeth, but not Macbeth.
(I liked the play Macbeth, but not the character of that name.)
Correct: Time magazine carried a review of Blade Runner, the film based on the novel Do Androids Dream of Electric Sheep?
(The periodical, film, and book title are all italicized or underlined. Note that the question mark is italicized also because it is part of the title.)
Titles of radio and television series as well as works of art are underlined or italicized.
Correct: Rodin's The Thinker
Correct: We used to watch reruns of Gilligan's Island.
Correct: My favorite Star Trek episode is "The Trouble with Tribbles."
(Note the last one--the series is italicized; the episode is in quotation marks.)
See also Underlining and Italicizing, Italicized Names, and Titles with Quotation Marks.
If an italicized or underlined name or title appears in the title of a work or some other writing which is otherwise italicized or underlined, the writer has a choice:
1. Normally the specific item reverts to standard type. This is always done in bibliographies and formal references.
Example: A Commentary on Piers Plowman2. Or you may italicize or underline the title or otherwise italicized or underlined writing without regard to the further italicized words. This may be necessary to avoid confusion.
(Book title contains name of another book)
Example: A Commentary on Piers Plowman helped me understand that medieval work.
(Using the style of #1 for this would be more likely to confuse the reader.)