The modern form of the piano, which emerged in the late 19th century, is a very different instrument from the pianos for which the classical literature for piano predating this time was originally composed. The modern piano has a heavy metal frame, thick strings made of top-grade steel, and a sturdy action with a substantial touch weight. These changes have created a piano with a powerful tone that carries well in large halls, and which produces notes with a very long sustain time. The contrast with earlier instruments, particularly those of the 18th century (with light wooden frames, lightly sprung actions, and short sustain time) is very noticeable. These changes in the piano have given rise to interpretive issues and controversies involving the performance on modern pianos of the music written for the earlier kinds of piano, particularly since recent decades have seen the revival of historical forms of the instrument for concert use.
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