A coupler is a small metal fitting that attaches the bag to the tip of your pastry brush. It makes it easier to manage and control the flow of flour, sugar and other ingredients while frosting or decorating a cake or cupcake. By attaching the bag directly to the tip of your brush, you avoid having to constantly squeeze and maneuver the bag in order to dispense the correct amount of frosting or color. Plus, it’s a handy way to keep those pesky tips from getting clogged up with icing. You can buy disposable pastry bags with or without couplers, but I recommend buying one so that you have it on hand when you need it.