In this paper, we propose Floating Band D2D, an adaptive framework to exploit the full potential of Device-to-Device (D2D) transmission modes. We show that inband and outband D2D modes exhibit different pros and cons in terms of complexity, interference, and spectral efficiency. Moreover, none of these modes is suitable as a one-size-fits-all solution for today's cellular networks, due to diverse network requirements and variable users' behavior. Therefore, we unveil the need for going beyond traditional single-band mode-selection schemes. Specifically, we model and formulate a general and adaptive multi-band mode selection problem, namely Floating Band D2D. The problem is NP-hard, so we propose simple yet effective heuristics. Our results show the superiority of the Floating Band D2D framework, which dramatically increases network utility and achieves near complete fairness.