The stradella bass system is a bass layout usually found in most piano and chromatic accordions. A full layout is arranged this way:

--A       E      B --  counter bass
 --F       C      G --  fundamental bass
  --Fmaj    Cmaj   Gmaj --  major chord 
   --Fm      Cmin   Gmin --  minor chord 
    --F7      C7     G7 --   seventh chord 
     --Fdim    Cdim   Gdim --  diminished chord

The first two rows play single notes, while the last four play chords, with adjacent buttons arranged in fifths (ie., the note after C is G, then D, A and so on). Only a single octave can be played, usually from C to B.

A full size instruments will have 120 buttons, starting (fundamental bass) typically from A to A#, therefore with 20 columns. As only an octave is available, in this layout there will be 8 repeated columns to allow easier chord switch when playing in some keys. Note that the real octave range may not (and usually is not) actually be from A to A#. This is just the way the buttons are arranged.

Smaller instruments may have 96 buttons (16 columns from B to G#) 80 buttons (same number of columns, but no diminished chord row), 72 buttons (12 columns from Db to F#), 60 buttons (12 columns as in 72 buttons, but no diminished chord row), 48 buttons (12 columns but no diminshed and seventh chord rows) or 24 buttons (8 columns from Eb to E and only fundamental bass and major and minor chord rows).