I have had generations of ThinkPad, since the x30 series. And the older older all had a very stiff metal (magnesium) frame as a core and usually a metal lid. Nowadays they went to very stiff plastic and carbon to save weight and thickness I guess. But they are much more portable because of this. I think Apple patented the "unibody design" at the time btw. The current company macbook pro I use for work is very heavy compared to other devices, because of all the metal and glass. It is quite a lot to carry around.