Stretcher bars form a frame on which to work embroidery, and serve the same purpose as embroidery hoops and scroll frames - they keep the fabric taunt while stitching. Stretcher bars are commonly used for canvas embroidery and needlepoint.
Stretcher bar sections are purchased in pairs, so you will need two sets of the required length to make one frame.
Assemble the bars by sliding the tongue and groove corners together to form a square or rectangular frame.
The embroidery fabric is then attached to the frame using tacks with large heads, while stretching the fabric slightly to make it taunt. The fabric will need to be periodically re-stretched on the bars if it starts to slack.