In 1804 William Southwell drew up a deed of partnership with his brother Nicholas and sons John (ii) and Francis, after which the business was known as 'William Southwell & Co.'. The partnership was 'to free himself [William] from the day-to-day pressure of general business and to concentrate purely on the design, development and manufacture of instruments and, in particular [...] the piano.' The partnership was due to last four years but was not dissolved until 1815 (De Valera).

[Dates differ according to sources: Hogan gives the date of dissolution as 1814 but De Valera states it was 1815.]