The new Betty Bridge was designed to solve the problem of the difference in height on your sewing machine's bed or surround, causing seam allowances to get caught and flip during sewing. To use the Betty Bridge, peel the back paper off and place the Betty Bridge on the bed or surround of your machine (sewing or serger). The surface that had the difference in height is now smooth! Seam allowances will glide through and not get caught and flip during sewing. * DO NOT PLACE OVER FEED-DOGS Some machines have a drop-in bobbin, or there may be a divot in front of the needle that could be a problem. The Betty Bridge will help with this also. Another use: When joining pieces together, make sure edges are lined up evenly. If the bottom piece is a light value and your sewing table is light, it will be difficult to establish alignment. Place the dark Betty Bridge under your pieces so alignment is easier to see. Each pack contains 2light and 2 dark repositionable Betty Bridges. Just peel backing off and stick down.