Ever since the first performance of Handel's Halleluiah Chorus, it has been customary for the audience to stand during the performance. This is because King George rose to his feet at the premiere of this oratorio-and the tradition has stuck with us ever since.
After years of listening to the Messiah, performing many choruses from it, and being in the audience countless times, it is something I pretty much do automatically.
I attended a live performance today. No one stood. It was weird.