The jack stud helps support the header above the door or window. The jack stud is the smaller inside board that connects to the bottom of the header.
Tips for Putting the Jack Stud in the Frame
First, you want to manage the weight. The framing that the jack stud is used in helps support the transfer of the structural weight over the opening and down to the floor.
Make your Mark
When installing the frame you will want to mark where the door or window opening will be.
Measure and Cut
Cut the jack stud so that the height is right for the opening of the door or window and then you will be able to nail it to the king stud. Installing both together takes less time and makes it easier.
You’ll then want to find the header length. This can be done by finding the distance between the king studs. For most single frames a pair of 2x10s will do the job.
Now you can nail the header into place and finish installing the frame.