In the 1790s composers such as Dussek were pressing for an increased compass for keyboard instruments, which had been five octaves for most of the 18th century. For square pianos Southwell had the ground-breaking idea of running the extra treble keys under the soundboard, with the hammers rising through a narrow gap at the back to strike the strings. He patented this development in 1794.
F.W. Southwell – Favourite Waltz