This is to show bone placement in an over-bust corset when it has front busk opening.
The yellow (A) position is usually only on a front busk opening corset, The flat steel bones can be placed behind the metal busk for extra front stiffness to hold the tummy flat.
Centre front bones can be added to a closed front corset in larger sizes, but we don’t usually feel they are needed to give a good shape, and can dig in when sitting down.