Standard stretcher bars 30 inches or longer have slots (mortises) in the center to accommodate center braces. This is also true of heavy duty stretcher bars 36 inches or longer. The assembly steps below are for a large stretcher frame with overlapping center braces. For smaller stretcher frames without center braces you can eliminate steps 2, 3, and 6. You can also watch the assembly in this video.
1. Lay out the stretcher bars with the bead facing up. Include any center braces.
2. Align the slots (dado joints) in overlapping center braces.
3. Insert the tabs (tenons) on the center braces into the slots (mortises) on the stretcher bars.
4. Align the dovetail keyways at the corners.
5. Tap dovetail keys into the four corners.
6. Turn frame over and screw together overlapping center braces.