The development of the modern piano was unique in music history. For the sake of comparison, the basic dimensions, materials, principles of construction, and manufacturing techniques of string instruments were well established by the latter part of the 16th century, so in all essential details, a violin made today differs little from one built 400 years ago. A modern piano, though, bears little resemblance to its antecedents, the harpsichord and the fortepiano, except for the obvious fact that it is operated by means of a system of keys and pedals.
 
Wolfgang Amadeus Mozart could never have guessed the dramatic innovations of a keyboard instrument upon which, say, a Romantic or modern piano concerto might be performed, any more than he could have imagined that such powerful music might actually be written one day. Fortunately, we know a great deal about the kind of instrument for which Mozart was writing his piano music, and many examples of Classical keyboard instruments still survive in playable condition. To set a clear historical perspective, as late as 1750, the most common keyboard instruments available were harpsichords and spinets, while in northern and mid-Germany, the clavichord enjoyed great popularity, while the fortepiano or Hammerklavier was still comparatively rare across Europe.
 
Cristofori 1722 The first recognizable pianos (an example from 1722, left) were built in Florence, by the Keeper of the Instruments of Music to the Court of Prince Ferdinand di Medici, Bartolomeo Cristofori. Known as the Gravicembalo col piano e forte ("Harpsichord with loud and soft"), it was far more than just a harpsichord variant, in which the string-plucking quills were replaced by a hammer system. Cristofori also developed an escapement device which, for the very first time, enabled the player to alter the dynamic levels by means of finger-pressure alone. But paradoxically, the novelty of the first true piano was short-lived. Cristofori was Italy's first and last great piano maker, but his massive achievement was commemorated in a handful of compositions which survive today. The earliest known pieces for fortepiano were a set of 12 sonatas written by Lodovico Giustini of Pistoia, issued in Florence during 1732 as Sonate di piano e forte detto volgarmente di martelletti, and dedicated to Prince Antonio of Portugal.
 
By 1750, composers such as Johann Joachim Quantz and Carl Philip Emmanuel Bach were writing for and about the fortepiano with a degree of enthusiasm that already suggested a fairly wide assimilation of the instruments within the German-speaking world. The Freiburg (later Dresden) keyboard instrument maker Gottfried Silbermann, one of the greatest of all organ-builders, first became aware of Cristofori's pioneering work in 1726, when he read a translation of Scipione Maffei's analytical review of the design and fabrication of the Cristofori pianos. He designed and built two experimental instruments, and soon hit upon a novel improvement which he termed as the Prellmechanik action, because of the use of a Prelleiste or "rebound-rail" within the mechanism. Silbermann's pianos, like Cristofori's, took the wing-like shape of a grand piano, an impressive free-standing piece of furniture in its own right.
 
Silbermann 1746 During 1736, Silbermann had the opportunity to demonstrate his fortepiano to the great Johann Sebastian Bach, who complained that the treble was too weak in tone and the keys were hard to play. Bach kept his long-standing allegiance to the harpsichord, and it is not hard to see why. Silbermann was offended, but not deterred, and Bach's comments can be seen in retrospect as a fairly accurate judgment of the limitations of piano technology as it existed at the time. At this point in the 18th century, the fortepiano had a sound which resembled the harpsichord's brittle timbre, and had nothing in common with the mighty sonority of a modern concert grand. As with the harpsichord, the frame was wooden, while the hammers were lighter and harder, and had to strike strings which were both thinner in gauge and fewer in number. During the next decade, Silbermann continued to develop and modify his instrument (right), and in 1746, he was invited to demonstrate the latest examples at the Potsdam Court, where the musical monarch Frederick the Great was so taken with them that he immediately placed an order for fifteen. Bach, meanwhile, was a fair and discerning critic, and when he next encountered Silbermann's pianos at the Court in Potsdam, he announced himself well pleased by the most recent refinements and improvements.
 
Of the great Classical masters, it was Franz Joseph Haydn in his 60-odd keyboard sonatas who shows most graphically how technical advances affected his own musical style. As Eva Badura-Skoda has observed in her essay Haydn, Mozart and Their Contemporaries (Keyboard Music, ed. Denis Matthews) "Haydn's first keyboard sonatas, with their short, suite-like movements, were still intimate in character. Soon, however, he was striving for more spacious formal construction, greater sonority and expression, and by the late piano sonatas, the dimensions have become Beethovenian...In 1788, Haydn wrote to his publisher Artaria, 'I have had to buy a new fortepiano in order to write you three clavier sonatas really well...' This remark could imply either that he did not own one before or, more probably, that an 'old' piano was no longer adequate."
 
Some commentators assert that the prime influence on Haydn's keyboard style was C.P.E. Bach, though this is not entirely accurate, for there is strong evidence to prove that the biggest influence was the Viennese composer Georg Christoph Wagenseil, a composer known personally to Haydn (Bach was not), and who made significant contributions himself to the advancement of sonata form. The historian Hermann Abert points out that there were also major geographical divisions at work. He claims the sonata evolved in two directions: North German, as evidenced by C.P.E. Bach, and South German/Austrian, represented by Wagenseil. While the former drew fundamentally on the Baroque dance suite, with its generic dances such as allemande, courante, gigue, etc., the latter was more affected by the fashionable Salzburg serenade genre, and thus the minuet found its place in the Classical keyboard sonata quite early in its evolution.
 
√Črard 1808Haydn knew all the keyboard instruments of his day, from the harpsichord to the early grand piano (an √Črard from 1808, left). Many musicologists, however, believe that the question of which keyboard to choose in performance is of secondary importance, because Haydn, like many of his contemporaries, did not specify a particular instrument.
 
Writing from Mannheim to her husband Leopold in Salzburg, Mozart's mother Anna commented on December 28, 1777, "Here Wolfgang's playing is very different from that in Salzburg, for here, there are pianos everywhere, and he handles them incomparably..." Shortly after settling in Vienna in 1781, Mozart obtained what was by the standards of the day a state-of-the-art fortepiano, whereas nothing comparable had ever been available to him previously in Salzburg. It seems puzzling that Salzburg lagged so far behind, but that Mozart was deeply impressed by what he had heard and experienced as a player of these new instruments is clear from many sources, and by now, the Classical piano really had come of age.