Using a coupler in your piping bag allows you to easily use different piping tips of the same size without having to change piping bags or empty out the contents of the bag.
How To Use
Couplers have 2 pieces - the base and the ring/washer which screws onto the base.
Insert the base into your piping bag and trim the piping bag (if necessary) so the hole at the end of the base is exposed.
Sit the piping tip over the outside of the base and place the washer over the tip. Tighten so that the piping tip is secure and doesn't move around.