Hey Avşin, This is a great idea, it can help imagine the ribcage in dimension. I can see that you were using boxes to help you construct the ribcage in perspective, that's a good plan. I did a quick little diagram to show you how I would do it. This is something that comes in handy when you have to do character turn arounds and make them all feel like they are inhabiting the same space. I hope it helps :)