La Traviata

When Alfredo first sings the "Di quell'amor", in "Un di, felice" (http://www.dlib.indiana.edu/variations/scores/bhr7293/index.html , p 40 ff.), at the syllable "cro-" (of "croce") he reaches the high "mi" note of the scale, which, in this case, is a high A, because we are in the key of F major. The same is true (albeit one octave higher) when Violetta repeats the tune (and words) as part of "A fors' è lui".

Later on, Alfredo sings the same "Amor, amor è palpito" tune from the balcony, in the middle of "Sempre Libera" (p. 66), which is in the key of A-flat major. In order to reach the "mi" note of the scale on "cro-" (of "croce"), the tenor would need to sing a high C. Either Verdi did not want to impose this on the tenor, or he had some other (artistic?) reason to not write in the high C (perhaps to diminish Alfredo's rôle here, since he's only on the sidelines), so he wrote in an G, which always sounds terribly unsatisfying to me.

But that's the whole point -- the tension between whether the tenor will sing the G or the high C! Most tenors should do the G, because most of them would sound bad on the high C. But there's an implicit high C there, because, of course, it's the same tune from before, which reached the "mi" note every time it was merely in the key of F.

And thus, one waits there to hear whether the tenor will or will not sing the higher note, and there's a time-period of tension, and then disappointment if he sings the G, but orgasmic release-of-tension if he sings the high C.

When I was growing up, we had a recording of Maria Callas and Alfredo Krauss, in which he does sing the high C. I looked on Youtube, and found no other recordings with the high C, but I did find the Callas/Krauss one: http://www.youtube.com/watch?v=vofYDpBKAhY