A concept of effective frequency is introduced to study dual-frequency (DF) capacitively coupled plasmas (CCP) which can be analyzed in a fashion similar to single-frequency (SF) CCP driven with effective parameters. Effective frequencies can be defined quantitatively for ion bombardment energy distribution functions and rf discharge parameters by analyzing ion dynamics in ion-collisionless dual rf sheaths and a homogeneous plasma model for dual rf discharges, respectively. Unlike the driving frequency in SF CCP, the effective frequency in DF CCP is dependent on the ratio of two driving voltages or currents. This characteristic makes it possible to control the ion flux and ion bombardment energy independently. The abrupt transition of the effective frequency leads to several physical phenomena unique in DF CCP (c) 2005 American Vacuum Society.