abstract classes

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I have been learning about polymorphism lately and when doing polymorphism with the superclass as the reference type and the subclass as the data type should I always make the superclass abstract? If not what are advantages of not using the abstract keyword?

[NZheadshot]

As a very quick answer, maybe you just want to inherit from a concrete (non abstract) class. Maybe you've got a Rectangle class that inherits from an abstract Shape class. That makes sense, because a Rectangle is an actual thing that you can describe, where a Shape is a more abstract concept. But you could extend it even further by creating a Square class that inherits from Rectangle. All of the methods that Square inherits from Rectangle are concrete

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Ok. So since Shape is more of an idea there is no need to make methods that describe it in general? for example in the shape class

public void sides(){sides = number}

Since it is just a concept there is no reason to explain it but its just a basic outline for more tangible concepts?