Mozart, Sonata K. 310, first page of the autograph. Via Wikimedia Commons (public domain).
Near the end of the first movement of almost every classical concerto, something strange happens: the orchestra stops on a suspended chord, the conductor lowers his arms, and the soloist is left alone, sometimes for minutes. This is the cadenza, and to grasp what it is you have to know what it was: the moment when composition broke off and improvisation began.
In the eighteenth century the convention was a clear contract. The orchestra arrived at that chord, a precise harmonic signal, a six-four that demands resolution, and there the soloist did his turn: he elaborated the movement’s themes in his own way, showed off the best of his technique, and with a conventional trill signalled the orchestra to come back in. Different every night, each performer’s own. Tosi recommended that it stay within a single breath; the virtuosos couldn’t have cared less even then. Mozart improvised his own in his piano concertos and wrote some down only for his pupils: the ones that survive are the most direct trace we have of Mozart the improviser.
Then composers took the territory back, and the tightening of the screws has a date and a name: Beethoven, Fifth Concerto, the Emperor, 1809. At the point where tradition allowed freedom, Beethoven writes into the score an explicit order not to play a cadenza, and sets down in notes exactly what the pianist must play. From then on the cadenza becomes text: Brahms writes his, Tchaikovsky writes his, Rachmaninoff writes two alternatives and leaves only the choice. For the older concertos left without a cadenza by the composer, a frontier trade is born: the cadenza written by someone else. Joachim’s for Brahms’s concerto, which was dedicated to him, and Kreisler’s for Beethoven have become almost part of the work, and carry a signature inside the signature.
Today things are opening up again: a generation of soloists has gone back to really improvising, or to writing cadenzas of their own, even provocative ones, and audiences are rediscovering the original thrill of the moment: not knowing what happens next. It is the small patch of anarchy that survived inside the museum, the point where the text willingly hands the keys to the performer. When that suspended chord arrives and the conductor lowers his arms, you are watching the last open frontier of classical music. Enjoy it: it lasts one trill.