Ali was sitting at her desk in the front of the room. On the board was the day’s lesson. Once students would come in she will go more in-depth—if they have questions that is.
Magic requires many elements, but a prime one is familiarity. That is why most spells are line-of-sight. The wizard must see what it is in order to charm/curse it. This has been said many times. But then how could someone use an Accio spell to summon their object when it wasn't within his/her sight? Well, two points. 1) They know their object inside and out, he/she is very 'in tune' with it so to speak; and 2) he knows exactly where it is. Now I would suppose that, even if he didn't know where it is, his familiarity with it would suffice (if he were powerful enough).
It is suspected that magic has three levels of power: Any wizard worth his salt can Accio a visible object, a more powerful wizard is needed to Accio a familiar and location-known object, but not visible, and only the most powerful can Accio an object on familiarity alone. For example, if a wizard saw a tablecloth over an obvious object underneath, that person could Accio it into his or her hand. Of course, this could be very dangerous. It could be a bowling ball with teeth or something.It would be different if there were an object in a tablecloth in a room not in view. Then there's nothing there as far as the person knows, hence no Accio.
HW: What object do you think you could summon by familiarity alone (DO NOT TEST THIS.) Why do you think you can? How dear do you think an object must be for you to actually summon it without seeing it?