The top plate is a long, thin box stuffed with straw that runs over the bales to attach the trusses. You can see the all-thread protruding through it. This comes all the way up from the foundation. The walls wobble until this top plate is in place, and after it's on, they don't budge! The best method is to construct this and have it ready on the day of your wall-raising, so you can use the crowd to hoist it in place. We didn't use this method and it was very heavy raising it into place. It should also be constructed to fit the trusses, not the bales, and we didn't use that method, either! Live and learn.