With minor modifications this is as explained by Alan Polivka in his Accordion FAQs

QUESTION:  What are the different tuning options available for an accordion?

Having spent several years researching, analyzing and of course, performing the tuning of piano accordions I have run across and/or tried quite a few variations.  There are almost as many different tunings for an accordion as there are accordion manufacturers and tuners.  Each tuning gives a different overall sound.  However, in general, the various accordion tunings fall into one of the categories discussed below. 

Note that throughout this article, I am only talking about tuning of the right-hand side of an accordion since typically there is no tuning variations encountered on the left-hand side of accordions. 

WARNING:  I do not recommend trying to tune your own accordion unless you are very experienced at it.  You can easily damage a good set of reeds and reed skins by not knowing what you’re doing.  Furthermore, if you plan to have your accordion tuned, be sure to get multiple references on the person who will do the tuning for you.  There are very few truly qualified accordion tuners in existence these days. 

Overall, an accordion may be tuned to standard A=440Hz pitch, or it may be tuned to something else (sometimes 442, 443 or 444 is used to make the accordion stand out a bit from other instruments).  However, one or more of the reed sets in the accordion may be intentionally de-tuned relative to the other sets to change the general sound of the accordion. 

First of all, the tuning is dependent on the reed arrangement in the accordion.  For a full sized professional accordion, there are typically four sets of reeds on the right hand side (a “4-reed accordion”).  There are two different reed arrangements commonly used in 4-reed accordions.  I’ll refer to them as “Reed Arrangement LMMM” and “Reed Arrangement LMMH”, defined as follows: 

“REED ARRANGEMENT LMMM” has one low octave set of reeds (analogous to 16’ voices in an organ), known as “bassoon” reeds.  It also has three sets of middle octave reeds (analogous to 8’ voices in an organ).  Often this arrangement is represented as shown below: 

“REED ARRANGEMENT LMMH” also has a set of bassoon reeds.  It only has two sets of middle octave reeds, however.  In place of the third set, there is a set of high octave (“piccolo”) reeds.  These are analogous to 4’ voices on an organ.  This arrangement is often represented as shown (envision a circle around the object shown below):

The typical full sized professional accordion is referred to as a “4/5accordion” because it has 4 sets of reeds on the right hand side and 5 sets of reeds on the left hand side.  Occasionally, you’ll run across an accordion with 5 sets of treble reeds (LMMMH) and/or 6 sets of bass reeds.  Less expensive and/or small accordions will typically have fewer reed sets than 4/5.

In all cases, the bassoon or L reeds, at least one set of middle octave reeds (referred to as the “clarinet” or M reeds), and the piccolo or H reeds (if present) are all tuned to the same standard (e.g. all are tuned to A=440 Hz). 

The way that accordions are made to sound different from one another is in the way the remaining set(s) of middle octave reeds are tuned.  Typically, they will be tuned a little bit off from the others to get a tremolo effect (a.k.a. a “beat note”).  The amount of tremolo (actually the rate of the tremolo) is typically referred to as the amount of “wetness”.  If all middle octave reed sets are tuned exactly the same (no offset), the accordion is said to have “dry” tuning.  In that case, no tremolo is heard.  

Another term sometimes used for a wet accordion is to call it a “musette” accordion.  However, this term has some ambiguity with it since many accordions have a shift button labeled “musette” even though the accordion may be tuned totally dry.  So, to avoid confusion, I’ll generally use the term “wet” rather than “musette” to refer to an accordion that has some reed sets intentionally de-tuned.

One of the fundamentals:  Tuning is often measured in “cents”.  1 cent = 1/100 of a half-step, or a difference in pitch by a factor of 2 raised to the 1/1,200 power = 1.000577789507.

 For REED ARRANGEMENT LMMM, following are some of the different tunings that I have encountered or done myself.  First, I’ll establish some terminology.  Let’s refer to the three sets of middle octave reeds (the “middle line”) as follows:

•           set #1 = “Clarinet” reeds; tuned to the same reference as bassoon reeds;

•           set #2 = “Violin” reeds; tuned sharp with respect to set #1;

•           set #3 = “Musette” reeds; tuned either sharper yet than set #2 or else flat with respect to set #1;

Note that the term “clarinet reeds” is fairly commonly used among accordionists and tuners to refer to set #1.  However the terms “musette reeds” and “violin reeds” (for the other middle octave reed sets) are often used interchangeably.

First of all note that some accordions have set #3 tuned sharp and others have that set tuned flat.  “French Musette” tuning is distinguished mainly by the fact that it is much wetter than all other tunings.  Furthermore, French Musette tuning typically has set #3 tuned flat by the exact same amount that set #2 is tuned sharp.  If you do a Fourier analysis of the result, you’ll find that this results in something similar to non-suppressed-carrier amplitude modulation (AM) but with harmonics (for any radio-electronics buffs out there).  (The 2-reed “musette” sound, as would be found in Reed Arrangement B, is analogous to suppressed-carrier AM).  For non-technocrats, this means that in either case, there is not a “blend” of tremolos but rather a single tremolo being applied to each note.

Another type of tuning that is popular in piano accordions is the following. This tuning has reed set #2 tuned just a little bit sharp and set #3 tuned a lot sharp.  The reason for this is so that the player has a choice of a fairly dry sound (by just selecting sets #1 and #2) or a wet sound, by selecting all 3 sets (or selecting #1 and #3).  Note that if the amount of offset is not consistent between sets #1-to-#2 vs. #2-to-#3, then you do end up with a “blend” of a tremolos in this tuning.

“Slovenian” style accordion tuning (I’m referring to accordions used in Slovenian style bands here in the US - I find that the true European accordionists often do things differently) typically has a moderately (but usually not totally) dry sound.  Most have the violin reed set tuned a little bit sharp. The Slovenian tunings vary a lot, however, in how the third (musette) set is tuned.  It is tuned sharp by some tuners and flat by others.

Frankie Yankovic, however, has his accordion tuned totally dry.  The key characteristic of Slovenian tuning is that there is a little bit of wetness in the lower notes, yet not so much in the higher octave notes.  This keeps the accordion from sounding as out of tune as some wetter tunings sound to some people.

“German” style tuning generally falls into the category of about ½ as wet as full French Musette.  This style of tuning is also sometimes referred to as “Polka” tuning or as “Continental” tuning.

For Italian music, one hears just about any of the above styles of tuning as well as variations that fall between those mentioned above.

For reed arrangement LMMM, there is one less set of middle octave reeds to play with.  Thus, there are fewer degrees of freedom for tuning the accordion.

The variations occur primarily in how sharp the violin reeds are tuned and secondarily, in whether they are tuned wet across the entire range of the keyboard, or dryer for higher notes (the latter being more of the “Slovenian style” tuning).

The bottom line in all of this is that you should play multiple accordions with the different types of tunings until you find one that you like.  Then either buy it, or borrow it and take it to your favorite tuner along with your accordion and ask him to tune yours just like it. 

CAUTION:  Make sure you use a reputable tuner.  Tuning accordions properly is much more complicated than most folks realize.  A good set of reeds can be ruined by an inexperienced tuner. 

Particular things to listen for while you’re trying the different accordions are the following:

• Do you like its violin or musette sound (i.e. the sound with all middle sets playing, which, incidentally, is not always marked “musette” on    the accordion shifts themselves)?

• Do you like the “master” sound (i.e. when all the reeds are played simultaneously).  Many folks like the real wet French sound. However, this does make the “master” sound too much out of tune for some of us, particularly on the higher notes of the keyboard.

• Do you want the feature of being able to select a dry sound or a wet sound in the same accordion?

• Do you want piccolo reeds (“Reed Arrangement LMMH”) or not?  If your  accordion has piccolo reeds, a good tuner/repairman can replace the piccolo reeds with musette reeds, if you prefer.  This is not a cheap endeavor, however, and it requires a very skillful repairman/tuner, and is commonly discouraged.

Remember, any accordion with at least two sets of middle octave reeds can be made into a “musette” accordion, simply as a matter of how it is tuned.  Now, if you want a full French Musette sound, you need three sets of middle octave reeds, tuned appropriately relative to one another, as discussed above.

Whether you want a full French Musette sound or just wet tuning of two sets of middle reeds is something you must decide after playing both types of accordions.  One of the drawbacks of a very wet tuning is that to some people, the accordion will sound out of tune when the master switch is selected, especially in the higher notes.

Note from Joe Regina - So now that you have read all this and you are still confused remember that these are the types of tremolo that most accordion manufacturers use: Jazz 4 cents,  American 8 cents,  German 14 cents,  Italian 18 cents,  French 24 cents.  But all of this only matters if you ordering your accordion rather then buying whatever is available. If you are ordering your accordion with a French Musette ask the  dealer what he will tune the three middle (clarinet) reeds as? The most common  French Musette tuning of the three middle (clarinet) reeds is:

 Low       Middle    High
A=436     A=440     A441.5

Here is an alternate view point on the subject by Anders H. Bakke

Live Examples of Musette

5 cents Musette

10 cents Musette

15 cents Musette

20 cents Musette

If you are purchasing an LMMH Accordion you can only choose ONE tuning depth of Musette.

If you purchase the LMMM with the default tuning you get three tuning combinations, some restrictions
however let me explain that one.