Could someone outline the basic idea of crutch construction for me? Does the crutch actually become a permanent part of the structure or is it removed later? Pictures would be nice cause I ain't too bright!

Also what are the other basic construction methods used in scratch-building scale models?. I know a lot of WWI models use a "truss" construction which basically just involves building two sides and then turning them into a box. In some cases, then, formers are added to the sides of the box to make rounded sides.