Unfortunately, I must start this section by pointing out that you should NOT expect to be able to adjust the fit of your accordion once and then be happy for the future. It takes a lot of fine adjustment to get the accordion in a perfect position over a reasonable period of time. The whole process of refining the fit of the accordion could take weeks as your body tells you that it is not happy with your positioning or you find that you cannot manipulate the bellows as you should. You cannot rush this stage I am afraid.
Your accordion should be connected to you in a reasonably tight manner. Some people can manage this using the shoulder straps alone and others (me included) have to use a back-strap. The left arm strap should be tight but have enough slack for you to reach every button.
My preference is to sit down when I play although I am willing to play some music standing up - generally for short periods of time. A 120-bass, 5-voice accordion is quite heavy. I use a drum throne without a back as this gives me the freedom to manipulate the instrument to the full and reach all the keys and buttons.
When you put the accordion on, the keyboard should be almost vertical. It should not be at an angle with the top notes of the keyboard trapped between your legs (you paid a lot of money for those keys so you want to be able to use them!). The bellows should rest lightly on your lap. Some people think this will wear both the bottom of the bellows and your clothes but I think that a well made accordion will only start to show wear after about 20 years. Even then, it is minor wear so I don't worry about it. Leg (or, perhaps, bellows) protectors are available but I don't bother with them. The accordion itself should be almost vertical - not tipped back towards you. If, from the side, people can see more than a slight glimmer of light between you and the accordion, it is not on tight enough.
The (almost) vertical line where the keyboard meets the front grill should be just to the left of the centre line of your body as you look forward. This means that you do not have to stretch your upper body to get round to the keyboard.
When you have this in place, make sure that you can manipulate the bellows. Press the air button and then open the bellows, allowing the natural weight of the bass-side to assist the opening and your left arm muscles either to pull the bellows harder (for a louder passage) or slow the opening for a quieter section. During this process, the lower edges of the bellows (the part on your lap) remain close together.
Now raise your left arm to close the bellows, again using the natural weight of the bass-side with the additional control of your left-arm muscles. Now, the top edges of the bellows remain close.
The bass strap should be sufficiently tight that when you change from push to pull, there is no audible delay in the music as your wrist moves from putting pressure on the body of the accordion to the strap. The strap should also be loose enough for you to reach every button on the bass side. On a full-sized accordion, it is often reasonable to have to angle your hand more than usual to reach the furthest buttons.
Once this feels comfortable it's time to check a few points;
I have to finish this section by repeating that this process can take a long time - often weeks - and is worth the effort as a correctly-fitted accordion will be playable for extended periods without pain. A badly-fitted accordion can do you a lot of damage.
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