The tenon is the part of the headjoint where it goes into the flute body. The tenon must fit very precisely. To make sure your new headjoint fits on your existing flute, we must know the tenon size to great accuracy. Usually, if I know make and model of your flute, that will do, or your repairperson can measure this for you. Once you ergonomic flute headjoint is ready, it might need some final fitting. Your flute repair tech has all the special tools for this, it is a small job.