The earliest pianoforte (made in the first two decades of the eighteenth century) had a compass of four and a half octaves, a little more than that of the ordinary clavichord. The pianoforte of Mozart’s time had five octaves, and Clementi added half an octave in 1793. By 1811 six and a half octaves had been reached, and in 1836 (about the time of the publication of Liszt’s first compositions, barring the youthful Études) there were seven, or seven and one-third, which have remained the standard ever since. During all this time piano makers had been endeavoring to increase the rigidity of the piano frame. This was partly to take care of the greater size due to the adding of bass strings, but chiefly to permit of greater tension. The quality and persistency of the vibration depends to a great extent on the tension of the strings. Other things being equal, the excellence of the tone increases (up to a certain limit) with the tension. This led gradually to the introduction of iron supports, and later to a solid cast iron or steel frame, though up to 1820 only wood was used in the body of the pianoforte, until the tension became so great and the pitch so high (for the sake of tonal brilliancy) that the wooden frame proved incapable of sustaining the strain. The average tension on each string is, in the modern piano, some one hundred and seventy-five pounds, and was up to recent times much higher. The present Steinway concert grand suffers a strain of more than twenty tons, and, under the higher pitch of former years, had to stand thirty. The weight of the instrument itself is half a ton.