Over time, piano playing became a more strenuous and muscle-taxing activity, as the force needed to depress the keys, as well as the length of key travel, was increased. The tonal range of the piano was also increased, from the five octaves of Mozart's day to the 7? (or even more) octaves found on modern pianos.

In the first part of this era, technological progress owed much to the English firm of Broadwood, which already had a strong reputation for the splendour and powerful tone of its harpsichords. Over time, the Broadwood instruments grew progressively larger, louder, and more robustly constructed. The Broadwood firm, which sent pianos to both Haydn and Beethoven, was the first to build pianos with a range of more than five octaves: five octaves and a fifth during the 1790s, six octaves by 1810 (in time for Beethoven to use the extra notes in his later works), and seven octaves by 1820. The Viennese makers followed these trends. The two schools, however, used different piano actions: the Broadwood one more robust, the Viennese more sensitive.

By the 1820s, the center of innovation had shifted to Paris, where the Érard firm manufactured pianos used by Chopin and Liszt. In 1821, Sébastien Érard invented the double escapement action, which permitted a note to be repeated even if the key had not yet risen to its maximum vertical position, a great benefit for rapid playing. When the invention became public, and as revised by Henri Herz, the double escapement action gradually became the standard action for grand pianos, and is used in all grand pianos currently produced.

Some other important technical innovations of this era include the following:

Use of three strings rather than two for all but the lower notes 
The iron frame, also called the "plate", sits atop the soundboard, and serves as the primary bulwark against the force of string tension. The iron frame was the ultimate solution to the problem of structural integrity as the strings were gradually made thicker, tenser, and more numerous (in a modern grand the total string tension can approach 20 tons). The single piece cast iron frame was patented in 1825 in Boston by Alpheus Babcock, combining the metal hitch pin plate (1821, claimed by Broadwood on behalf of Samuel Hervé) and resisting bars (Thom and Allen, 1820, but also claimed by Broadwood and Érard). Babcock later worked for the Chickering & Mackays firm which patented the first full iron frame for grand pianos (1843). Composite forged metal frames were preferred by many European makers until the American system was fully adopted by the early 20th century. 
Felt hammer coverings, first introduced by Henri Pape in 1826, gradually replaced skillfully layered leather hammers, the more consistent material permitted wider dynamic ranges as hammer weights and string tensions increased. 
The sostenuto pedal (see below), invented in 1844 by Jean Louis Boisselot and improved by the Steinway firm in 1874. 
The over strung scale, also called "cross-stringing"; the strings are placed in a vertically overlapping slanted arrangement, with two heights of bridges on the soundboard, rather than just one. This permits larger, but not necessarily longer, strings to fit within the case of the piano. Over stringing was invented by Jean-Henri Pape during the 1820s, and first patented for use in grand pianos in the United States by Henry Steinway Jr. in 1859. 
Duplexes’’’ or aliquot scales; In 1872 Theodore Steinway patented a system to control different components of string vibrations by tuning their secondary parts in octave relationships with the sounding lengths. Similar systems developed by Blüthner (1872), as well as Taskin (1788), and Collard (1821) used more distinctly ringing undamped vibrations to modify tone. 
Today's upright, grand, and concert grand pianos attained their present forms by the end of the 19th century. Improvements have been made in manufacturing processes, and many individual details of the instrument continue to receive attention (see Innovations in the Piano).

Some early pianos had shapes and designs that are no longer in use.
The square piano had horizontal strings arranged diagonally across the rectangular case above the hammers and with the keyboard set in the long side, it is variously attributed to Silbermann and Frederici and was improved by Petzold and Babcock. Built in quantity through the 1890s (in the United States), Steinway's celebrated iron framed over strung squares were more than two and a half times the size of Zumpe's wood framed instruments that were successful a century before, their overwhelming popularity was due to inexpensive construction and price, with performance and sonority frequently restricted by single actions and double stringing