Ateco makes an especially interesting line of tips for piping "frill borders." The first time I piped one of these, it was actually mistaken for fondant. I piped the above ruffle with this tip, the 040, using medium consistency buttercream icing:turntable.
When I piped the ruffle on this cake, I kept about a third of the tip's length over the top edge of the cake, which allowed the ruffle to spill over the edge and flare out over the drop strings beneath it.
The frill border tip adds a surprising amount of depth and sense of motion. It's hard to imagine ever piping a shell border again! By the way, Happy Birthday, Nana! I hope you enjoyed your cake!