Ever since Stampin’ Up! included the Scallop Circle Bigz die in its catalog, stampers have been using this shape for card fronts and backs. The problem is how to connect them!
But sometimes you really want the whole circle. So my solution is to cut two complete circles and score one of them so that six scallops are above the score. This is the card back. I adhere that flap to the inside of the front, creating a type of easel. This allows a totally flat, complete front.
Using six scallops for the flap means that the bottom of the card will be resting on two scallops, not one, so it won’t rock. You can see this in the top picture here.