My home has several corner window pairs with a 4x4 corner post. In this case, the window sills were rotting and it turned out that the corner post was rotting as well. I installed the jack to make sure the project didn't increase in scope due to any drop in the top of the window frames. Looks like I'm going to have to learn plaster repair next. Amazingly, the plaster and paint tested free of asbestos and lead, as did the vermiculite in the attic.