Crossovers often occur in an official capacity in order for the property rights holders to reap the financial reward of combining two or more popular, established properties. In other cases, the crossover can serve to introduce a new concept that derives from an older one.
Crossovers generally occur between properties owned by a single holder, but they can more rarely involve properties from different holders, provided that the inherent legal obstacles can be overcome. They may also involve using characters that have passed into the public domain with those that currently enjoy copyright protection.
A crossover story may try to explain its own reason for the crossover, such as "they live next door" (one example being the casts from Golden Girls and Empty Nest) or "a dimensional rift brought them together" (a common explanation for science fiction properties that have different owners). Some crossovers are not explained at all. Others are absurd or simply impossible within the fictional setting, and have to be ignored by the series' respective continuities. Still others intentionally make the relations between two or more fictional universes confusing, as with Hercules and Aladdin, where each show is fiction in the other. Others can be from the same company, making it legal. Examples of this are Final Fantasy and Kingdom Hearts which are both from Square Enix, and CSI:NY and Miami.