"an eternal chain, linked together by cause and effect"

When this exists, that comes to be. With the arising of this, that arises. When this does not exist, that does not come to be. With the cessation of this, that ceases.Samyutta Nikaya 12.61

The Twelve Nidānas trace the origin of the endless cycle of  action and causation that leads to birth-rebirth and hence suffering in our current and past lives. Read a commentary on the Twelve Nidanas by Theodore de Macedo Soares.

Soares defines the Twelve Nidānas as, "Twelve links in the chain of the causation of samsara. The twelve nidanas are usually depicted in Tibetan Thankas as the ‘Wheel of Life’ drawn with twelve scenes forming a circle. In the center of the circle, passion, aggression and ignorance, usually depicted by a chicken, snake and a pig respectively, represent the basic pull--push--ignore dynamic intrinsic of a dualistic ‘I’ and ‘other‘ relationship. As the ego and its projections need constant maintenance, the nidanas constantly spin."