How should i use VirtualScreenLeft and VirtualScreenTop

In my application i need to know size of all monitors combined space, to adjust size of the window if it goes to big.

I found VirtualScreenWidth and VirtualScreenHeight in System.Windows.SystemParameters and it seems like what i need. But also i wonder what is VirtualScreenLeft and VirtualScreenTop and how i should take it in consideration? In which scenarios this values are set to not 0?