Incidentally this is wrong. The line is from a movie adaptation, not the original play. And at least in the song (which came still later), “plain” wasn’t singled out; there are five copies of that vowel in the phrase and she couldn’t get any of them at first. https://en.wikipedia.org/wiki/The_Rain_in_Spain
Incidentally this is wrong. The line is from a movie adaptation, not the original play. And at least in the song (which came still later), “plain” wasn’t singled out; there are five copies of that vowel in the phrase and she couldn’t get any of them at first. https://en.wikipedia.org/wiki/The_Rain_in_Spain