Endband cores are lengths of material, both flexible and, occasionally, inflexible, which are either sewn to the head and tail of the spine of a bookblock in a worked endband, or are incorporated into a stuck-on endband. They can be single or multiple but they should be described individually as separate endband components. They can be made out of a variety of materials and can extend onto the boards where they can be fastened through a variety of techniques or can be cut straight to the bookblock spine edge. Endband cores which extend and are fastened to the boards play a crucial role in the reinforcement of the attachment between the boards and the bookblock especially so in Greek-style bindings. Their number can vary from one to as many as 7 or even more according to the sewing technique used. A core may consist of a single cord or other material length, or from multiple lengths of the same material, for example a bundle of thin threads used together as to form a single cord. They are described starting from the core immediately in contact with the bookblock edge at head or tail, proceeding outwards.