Many cabinets contain parallel input/outputs on the rear panel, so that one speaker cab can be plugged into the amp head, and then a second cabinet can be plugged into the first cabinet; as this "daisy-chaining" approach is wired in parallel, plugging in a second cabinet lowers the impedance seen by the amplifier. Cabinets typically have an impedance rating printed on the rear panel, such as "8 ohm minimum" or "4 ohms minimum". These warnings mean that the user should not connect an amplifier requiring a higher impedance; tube amps in particular are designed to work with speakers having some particular impedance. Since speaker impedances as seen by the amplifier vary with frequency, with voice coil construction, and with the cabinet's acoustical loading (which also varies with frequency) there can be no exact match under all conditions. When users daisy-chain a second cabinet with paralled speakers, they must ensure the amplifier can handle the lower impedance it will see.