Love at first Crack! In BDSM a “single-tail” is a single-lashed whip like a bull whip or a signal whip that is used by dog-sled riders. For those of you who are new to whips and are looking for your first single tail, you should know that the 4 foot whip is the most popular and multipurpose size. Using a single tail allows the enthusiast a number of choices – to make a spider web of welts, to brush lightly as if creating a caress or cutting to draw blood. Accuracy, thunderous whip cracking and the ability to use in indoor play spaces, all add up to one versatile whip!