Paul MacKay writes:

>why is it forbidden to clap after each movement?...

Maybe because Amy Vanderbilt ruled so.  In her _Complete Book of
Etiquette/The Guide to Gracious Living_, of which the edition I own
dates back to 1958, (when you still needed the thing) she writes:

   At concerts the applause is held, even after a solo, until the
   conductor, by turning on the podium toward the audience, indicates
   that the selection is over. Even at the end of the program the
   enchantment should never be broken by applause until the conductor
   has turned for his bow to the audience. His each appearance from the
   wings is applauded, however, but the house becomes quiet the minute
   he turns to face the orchestra...

...and...

   It is proper at the opera to applaud after arias--the "claque" usually
   indicates when--and of course at each curtain. Entrances should not be
   applauded--but sometimes are by the overenthusiastic who thus break the
   spell of introductory measures.

Noting that Ms Vanderbilt elsewhere in her book spells Riesling (the wine
grape), reisling, one is free to take her adviceling,or leave it.

Denis Fodor                             Internet:[log in to unmask]