The Chain on a Jacobin consists of the feathers that make up the front half of the head formation. If you were to draw a vertical line through the center of the rose, all the feathers radiating to the front of the bird from the neck up become part of the chain. If you were to then draw another horizontal line through the rose, the chain feathers below this line become the lower chain extension and above are the upper chain extension.
What our standard calls for is as much feather length and mass extending out in front of the birds face as possible as long as the bird is able to carry and chain properly, otherwise it becomes a fault. The shape and length of the chain should match the mane in profile to give a balanced appearance.
When viewed from the side, in profile, the chain begins at the front of the neck and should then extend upward and outward until it reaches the level of the eye. From this point the chain should then sweep up and back until it passes by each side of the Jacobin's face, just past the eyes where blends in to become part of the hood. The narrowest point of the chain should the be at the base of the neck and the widest should be straight out in front of the face. This widest point is often referred to as "face feather" or "extension out in front of the eye". When mirrored by the mane the entire formation should the give the shape of an inverted pear or light bulb. A common fault we see is when the feathers tend to shorten too much as they begin to sweep up and back from the eye line giving a flat spot out in front of the forehead. This takes away from the sweeping, symmetrical curve we desire. The feathers of the Chain should not droop or sag. They should be hard and smooth and tight fitting. Good chain feather will extend straight out and then gently curve in at the ends towards the opposite side.
When viewed from the front the feathers of the chain should sweep forward from each side of the neck and come together and touch at the front of the neck. As the chain feathers radiate upwards the two sides should then gradually separate below the face and sweep back past each side of the face to become part of the stay feather that holds the hood in place. It is important that the feathers of the chain have the proper gentle roll inwards at the ends so that they cup the face nicely but not so much that they completely come together impairing the birds vision causing the bird to crouch and not stand up and show properly. When the feather lacks the proper roll at the ends and is too straight, the sides of the chain will often open up too much, completely exposing the face. With these birds the feathers at the base of the chain often do not come together as desired. We refer to these birds as "open faced". Still viewing the Jacobin from the front the feathers of each side of the chain must be hard and tight fitting. This allows the feathers on each side of the chain to interconnect and creates a nice even "knife edge", a continuous unbroken sharp line of feathers that sweeps all the way up and back from the neck, past the face to the hood. Still looking at the bird from the front, when the feathers of the chain do not interlock properly they will create a jagged, rough appearance. We refer to this as "shingling" and it is a major fault. Sometimes a bird will have only a single break in the chain rather than the entire shingled appearance. Such breaks can be caused by the presence of pin feathers or from the bird getting its chain caught on the wire of the judging pen. A good judge will fix such a break and see if it holds before penalizing the bird for that fault. However such breaks often occur at the very back of the chain where it meets the hood. It is at this point that the feathers at the back of the head which give support to the hood (often referred to as the spike) can sometimes grow to one side pushing into the chain causing it to break and not blend into the hood properly.
Describing Proper Chain Structure in a Jacobin
by Clint Robertson
This picture of a Yellow cock shows nice chain structure and how it slightly opens to expose the face.