Simultaneous supply and threshold voltage tuning has a strong impact on the energy reduction of LSI circuits. Therefore, techniques for optimizing the supply and threshold voltages simultaneously under a specific delay constraint of LSI circuits are widely investigated over the past 15 years. However, only a few previous work investigate techniques for effectively reducing the energy consumption of circuits with a small number of different supply and threshold voltages. In this paper, we present several interesting properties which are very useful for reducing the energy consumption of circuits with a small number of different supply and threshold voltages. Those properties are numerically explained using analytical delay and energy consumption models. Through circuit simulation using a commercial 28nm process technology model, we demonstrate that the properties presented in this paper hold for actual LSI circuits.