Overridable?

Protected Overridable Sub Whatever()
...
End Sub

Can anyone tell me what exactely the "Overridable" keyword is used for? What does it do and why would someone use it?

a good example of how an override sub works is this... looking at the form's KeyPreview and catching the keyboard buttons that you cant see getting pressed on keydown ( left , right , up , down ), override gives you more control over whats happening and allows you to prevent things happening ( True / False )

VB Code:

[color=blue]Protected[/color] [color=blue]Overrides Function[/color] ProcessKeyPreview([color=blue]ByRef[/color] m [color=blue]As[/color] System.Windows.Forms.Message) [color=blue]As Boolean[/color]
        [color=blue]Select Case[/color] m.WParam.ToInt32
            [color=blue]Case[/color] 37 '/// left arrow
                '// do stuff
            [color=blue]Case[/color] 38 '/// up arrow
                '// do stuff
            [color=blue]Case[/color] 39 '/// right arrow
                '/// do stuff
            [color=blue]Case[/color] 40 '/// down arrow
                '/// do stuff
        [color=blue]End Select[/color]
        [color=blue]Return True[/color]
    [color=blue]End Function[/color]

When you create a class which inherits from the class with the overridable method/function, you can override this method/function using 'overrides', the inherited class can have its own implementation of that method/function.