For beginners who are discovering this document, the remarks in the beginning are essential:
- seating position on a chair
- the shoulder straps + horizontal connecting backstrap
- the legs in a 90° angle
- both feet flat on the ground
etc
There is however, one element I personally would also note down in the text. It's clear from the pictures, but perhaps a written statement would make it clear:
The total weight of the accordion should always rest on the upper left thigh and knee.
If your accordion (heavyweight or lightweight) is hanging and your back is carrying the weight of the accordion (even when firmly attached with vertial shoulder straps + horizontal backstrap), you're in for big trouble sooner or later.
The back should never ever carry the weight of the instrument. The entire weight should be on the legs and knees.
This is THE most common mistake beginners make: they make their back carry all the weight. In music school, accordion teachers immediately correct this from the very moment they observe this behaviour.
Obviously I'm talking about playing in a seated position on a chair.
When you're playing in standing position, the horizontal backstrap is very important to avoid shoulder issues. The shoulder straps could tend to slip from your shoulders.