To track whether a mouse button was pressed, you must add an object field to the form object. When you add a component to a form, Delphi adds a field that represents that component to the form object, so that you can refer to the component by the name of its field. You can also add your own fields to forms by editing the type declaration in the form unit's header file. 

In the following example, the form needs to track whether the user has pressed a mouse button. To do that, it adds a Boolean field and sets its value when the user presses the mouse button. 

To add a field to an object, edit the object's type definition, specifying the field identifier and type after the public directive at the bottom of the declaration. 

Delphi "owns" any declarations before the public directive: that's where it puts the fields that represent controls and the methods that respond to events.