The Spirit core library functions around a simulation state.
The State is the object that holds every information needed for the simulation of the spin system. Currently a State can contain only one Chain which contains all the images (spin systems) of the simulation. One can easily understand the structure by looking at the following diagram:
+-----------------------------+
| State |
| +-------------------------+ |
| | Chain | |
| | +--------------------+ | |
| | | 0th System ≡ Image | | |
| | +--------------------+ | |
| | +--------------------+ | |
| | | 1st System ≡ Image | | |
| | +--------------------+ | |
| | . | |
| | . | |
| | . | |
| | +--------------------+ | |
| | | Nth System ≡ Image | | |
| | +--------------------+ | |
| +-------------------------+ |
+-----------------------------+