Village Chaining is the process by which one can move village centers abnormally close to, or within, each other. This is a process that is commonly used in making highly efficient and compact iron farms. Village Chaining was first discovered back in 1.4, and has been used ever since. The process was originally discovered by TangoTek, a member of the HermitCraft Vanilla SMP server.
1.7 and Below
The process in which village merging worked in 1.7 was that doors conjoin with the oldest village within range before creating a new village.
1.8 and Above
In 1.8 and above, the process by which villages merge work has been changed such that doors now link up to the village center closest to them, as opposed to the process in 1.7. Old Iron Farms no longer can be constructed, however, if kept loaded, the farms can still function until unloaded. This allows for the iron trench or iron foundry to be built in 1.7, and still function in 1.8