|Apparently not everyone stands for the "Hallelujah"|
(Josh Reynolds for The Boston Globe)
The usual explanation is that King George the II stood when he first heard it at the London premiere in March of 1743 and everybody else followed suit because, hey, he was the king. It's a great story with only one little flaw: there's no evidence that George II ever attended a performance of Messiah at all. The story appears to come, not from a contemporary account, but (according to Matthew Guerrieri in a 2009 article for the Boston Globe, whence cometh the picture above) from a secondhand description in a letter written by James Beattie 37 years later (a classic example of how urban legends originate). The story is, in short, almost certainly apocryphal.
The tradition appears to go back a long way, though. When George Harris attended a Messiah performance in 1750 he observed that "[a]t some of the chorus's the company stood up," suggesting that the custom extended beyond just the "Hallelujah." Six years later, another account mentions the audience standing for "grand choruses." In his video series on Messiah Andrew Megill, Music Director of Masterwork Chorus, describes a letter written by a woman who attended a Messiah in Handel's time complaining of audience members who weren't standing during the appropriate choruses—suggesting that the practice was already fairly well established.
The bottom line, though, is that nobody really seems to know where the custom originated or, for that matter, why so many of us are still doing it. Like the Christmas performance tradition, it seems to be a meme that just won't die. For anyone attending Messiah for the first time, it must seem just another example of the sometimes baffling and contradictory rules of etiquette that go with classical music concerts.
The Essentials: Bernard Labadie conducts the St. Louis Symphony Orchestra and Chorus, with soloists Lydia Teuscher, soprano; Allyson McHardy, mezzo-soprano; Jeremy Ovenden, tenor; and Kyle Ketelsen, bass-baritone, in Handel's Messiah. Performances are Thursday through Saturday at 8 p.m. and Sunday at 3 p.m., December 3-6. The concerts take place at Powell Symphony Hall, 718 North Grand in Grand Center. For more information: stlsymphony.org.