I recently purchased a big collection of concertina sheet music, only to discover that it was all written in tablature for a Chemnitzer (I play English). Took me a while to hunt down enough info to prepare a Rosetta stone for its beastly layout. In this and the next two messages I'll share the fruits of my labors.

First, the button layout. The instrument is generally divided into treble (right hand) and bass (left hand), though there is much overlap between the two sides. The treble has three long rows, arched as on an Anglo concertina. The bass has four shorter rows, similarly arched.

Chemnitzer tablature combines four different methods to label buttons:
(1) Simple numbers (0 to 18).
(2) Proximal number pairs, used on the left hand outer row: 2/3 means the outer button lying closest to buttons 2 and 3 one row in.
(3) Supplemental number pairs, used for buttons toward the top ends of rows. 2/0 (the second number is always zero) means the second button in this set, though the ordering is inconsistent between left and right sides!
(4) Special symbols, used for some of the top buttons. These symbols are "+", "*" and a circle with a slightly larger plus superimposed, like a sight with crosshairs; below, I use "+o+" as a substitute for this text-unfriendly symbol.

Note: these are just my terms, since I could find no exposition on the layout that would give "official" terms. I also refer to "+o+" as "home" because it plays the tonic of the primary key for the instrument. Published music assumes a C instrument.

Chemnitzers play different notes on the draw and pull. I have yet to make much sense of the pitch layout, and I probably won't try. It looks more like someone threw darts to decide which pitches go where.

The tablature uses a single treble staff line. The treble (right hand) button numbering is written above the staff; the bass (left hand) numbering is written below the staff, usually in a boxed area. The bare melody is duplicated on the staff in normal notation.

A draw is notated with a large circumflex over the treble side numbering (which may use dashes when only the bass plays). In bass-only runs, the draws may be notated in the bass area.

Standard tablature doesn't indicate which fingers to use.

BUTTON LAYOUT
  Row order: button #, press pitch, draw pitch
  L-to-R: top buttons to bottom buttons on instrument
  
  Left hand (bass):     (outward)
           1/2     2/3     3/4     4/0     +
  (btm)    AA      G       Eb      f       Bb        (top)
           D       BBb     c       C#      F#
      1       2       3       4       0       2/0
      DD      A       c       e       CC      GG
      GGG     G       B       d       f       FF
  5       6       7       8       9       1/0     0/0
  EE      G#      B       d       f#      F#      F
  AAA     E       A       c#      e       Bb      G#
      10      11      12      13      14      3/0
      BBB     f#      a       eb      c#      C#
      EE      e       g#      B       F       C
                       (nearest palm)
  
  Right hand (treble):                (outward)
      +       2/0     1/0     1       2       3       4       0       16
      D       g       G       F#      A       c       e       cc      ee     (btm)
      F       c       F#      G       B       d       g       f       cc
  *       4/0     3/0     5       6       7       8       9       15      17
  F       E       Bb      G#      B       d       f#      g#      b       ff#
  D       G#      E       A       c#      e       a       cc#     bb      ff
      +o+     6/0     5/0     10      11      12      13      14      17
      C       f       c#      eb      Eb      a       cc#     dd      bb
      C#      eb      f#      Bb      g#      b       ee      dd      eeb
                                  (nearest palm)