I'm putting a 60 x 40 vinyl replacement window in the space where I removed a steel & glass single pane. The installation is in the basement, which has concrete block walls. I've done all the major framing with 2x6 pressure treated lumber. I can put the window between 2 king studs, without jack studs. I won't be using any cripple studs because the new window goes all the way to the sill. But I see jack studs in all the how-to books. What is their purpose? Will the 2x6's twist without them, or something? Thanks.