There will be a chain pulling your elbows forward, and a chain pulling them backward. Together they ensure that you cannot move your elbows.
The chest chain can be connected to the rings around the elbow chains with screwlinks or with key rings. In addition it needs to be connected to a chain around your neck. If the neck chain would not be there, the chest chain would end up too low on your body, in the weak part of your belly. The belly is so weak that it is always possible to move the elbows backward. Making the chain more tight is very unpleasant for the stomach, and still leaves too much freedom.
These problems are solved by adding a chain around your neck that pulls the chest chain upward. The neck chain ensures that the chest chain stays at the chest. Since it touches only your neck (and not the throat), there is no risk of breathing problems.
On the picture below is the construction that I use. It is constructed from three chains. To the left is the neck chain. On the right side is the chest chain, which actually consists of two separate chains. Everything is held together by the key ring in the middle. The bigger rings lying separate, belong to the elbow chains. The purpose of the small key rings (connected to the big rings) is to make it easier to connect the chest chain to the elbow chains.
The length of the neck chain should be such that its end is at the point where the bone ends (or a few centimeters lower). Making it shorter will cause unnecessary pressure on the neck, making it longer causes the chest chain to press on the stomach.
The length of the chest chain should be such that your elbows are at the position where would they naturally be when your hands are tied behind your back Forcing them at another position is unpleasant and causes circulation problems. If the neck chain causes too much pressure, you should probably make the check chain longer.
