Backward compatibility

Backward compatibility refers to an important goal in the evolution of a system: if, after a change to an interface or protocol, a modified component on one side can still operate with an un-modified one on the other, the change is said to be 'backward compatible'.

Even though, without being changed, the un-modified one generally will not be able to make use of the (usually) expanded capabilities offered by the changed interface, backward compatibility is still a very important property for interface changes: without it, there will have to be a 'flag day' in which all components using that interface will have to be changed at the same time.