Diatonic accordions consist of a melody side keyboard with multiple rows that produce notes from a diatonic scale, while some models feature bisonoric bass buttons to provide rhythm and harmony.
Pressing an accordionist button releases air through an opened valve and vibrates free metal reeds inside, producing its unique sound.
Keyboard
Diatonic button accordions (or melodeons) consist of a melody-side keyboard with one or more rows of diatonic scale buttons that produce notes in diatonic scale; there is also an accidentals row and bass buttons on the left side.
Some diatonic one-row melodeons use what’s known as a four-apart layout, where each button plays the same note when pushing or pulling on the bellows. Most, however, use five-apart systems where each time the bellows are pushed a different button on the right hand keyboard alternates between playing major and minor triads of its home key key when being pushed by pressing on its bellows.
Large two-row diatonics used by Scottish accordionist Jimmy Shand and Irish accordionist Peter Pot feature a bisonoric keyboard, where each button plays two distinct notes when the bellows is pushed or pulled, giving this type of melodeon the flexibility to perform chromatic music as well as various diatonic styles. Furthermore, this type of instrument comes equipped with standard system left hand bass buttons; when pressed they sound the fundamental and seventh notes from a major triad chord.
Reeds
To create sound, the accordionist presses a button on either keyboard to activate a valve that controls air flow to vibrating metal reeds and produce sounds. The number of notes played per voice (accordions can accommodate one to eight voices).
Melodeons are small diatonic button accordions featuring rows of treble buttons for melody and bass notes or chords for accompaniment, commonly used in traditional folk music as well as modern offshoots like country and Irish music.
A chromatic button accordion can play any key on its treble side and typically utilizes either the Stradella bass system, free bass system, Belgian system or French system on the left-hand side. It features many reeds that sound a note when key is pressed; these reeds usually come equipped with Helikon basse reeds that give off tuba-like tones.
Bass buttons
Diatonic accordions feature two sides – a treble side played with fingers from your right hand and a bass/chord side played by using your left hand – that are separated by bellows that can be pulled and pushed.
Diatonic instruments use a system of buttons which produce two notes each, so every key on a melody row produces two tones within its major triad in its home key. Furthermore, when one pushes one of these buttons while playing in a flat key the notes produced will differ than when played sharp key.
Some accordions utilize a semitone-apart system in which buttons are arranged so as to produce all notes of the chromatic scale in one row, yet technically these instruments would not be considered diatonic as each button produces both chords when pushed and individual notes when pulled; nonetheless, most accordions with such arrangements can only produce simple melodies, and therefore are often used more for accompaniment rather than lead playing.
Sound
Melody-side keyboards consist of rows of buttons which produce diatonic scale notes. On the bass-side keyboards are pairs of buttons; when you push or pull on them they change notes which gives melodeons their distinctive sound.
Two-row melodeons are often tuned B/C (sometimes known as chromatic). This system covers all notes of the chromatic scale but requires considerable effort when playing certain key centres; additionally, large bass systems such as 18-bass three-row instruments or 120-button Stradella basses may make the instrument quite heavy for players.
Some diatonic accordions with one and two row boxes tuned a semitone apart (eg CG,DG etc) typically feature “bisonoric basses”, which play different notes depending on whether you push or pull on the bellows – you can see this demonstrated on YouTube videos by melodeon legend Jimmy Shand and also Scottish accordion champion Brandon Mcphee.