why concrete class must implement abstract method while abstract class need not

yes, just as the title...why concrete class must implement abstract method while abstract class need not. Here we have two cases, one is concrete class extends abstract class, the other is abstract class extends abstract class.the reason why concrete class must implement abstract method, i think it is if concrete class doesn't implement abstract method which it inherited from its abstract father class, then the method it inherited is abstract which obviously is not permitted in a thorough concrete class. i.e. it doesn't make sense that a concrete class contains the abstract part. while the abstract subclass can.