Duet concertina

The most common key layouts within the Duet system are:

• Maccann system (or McCann), the most widely produced vintage Duet system, an improvement on Wheatstone's earlier Duette system, patented by "Professor" John Hill Maccann in 1884 and licensed to Lachenal & Co.^[6]
• Crane system. Patented in 1896,^[7] this system was originally named the “English Combination Concertina” by the musical instrument retailer Crane & Sons Ltd. In 1912, it was adopted by the Salvation Army under the name “Triumph”^[8] and is said to be "easier to learn than the McCann or Jeffries".^[9]
• Jeffries system, less common than the above, and though unisonoric the layout of the buttons resembles that of the Anglo concertina.^[10] The distribution of notes is described as "splendidly haphazard".^[11]

• Hayden system, invented in 1963 and patented in 1986, is an isomorphic system in which all scales and intervals are arranged uniformly.^[12] Years after its invention, it was discovered that a nearly identical layout had been patented by the Swiss designer Kaspar Wicki in 1896.^[13] Though a more recent development, the majority of newly produced Duet concertinas since the 1980s are in the Hayden system.

There are a number of other types, far less common: a 1983 article notes patents including "Sharp's 1890, Hank's 1896, Huish's 1901, and a number of Patents by Dr. Pitt-Taylor between 1916 and 1924."^[14] Duet concertina designer Brian Hayden has also noted the Linton, Chidley, and Piano systems, the last including variants such as the Rust system and Jedcertina.^[15] From 1951, Wheatstone made a small number of instruments in the Chidley system, which superficially resembles the Maccann system, but has a more regular note pattern.^[16]

Duet concertinas are held by placing the hands through a leather strap, with thumbs outside the strap and palms resting on wooden bars. The largest duets play bass notes down to C below the stave, and a competent performer can play solo piano music with little compromise.^{[citation needed]}