have multiple 8" speakers (e.g., the 8x8" cabinet). Cabinets with mixed-size speakers are less common, but they are used (e.g., a bass amplifier cabinet with a 15" loudspeaker for lower frequencies and a smaller speaker for mid- to high-range frequencies. A cabinet is usually mono, but may have two inputs for a "stereo" amplifier. Two speakers in a cabinet may be wired in parallel (lowering the impedance) or in series (increasing the impedance). Larger multiples will usually be series/parallel to maintain an impedance of 4 to 8 ohms. 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 paralleled speakers, they must ensure the amplifier can handle the lower impedance it will see. Bass guitar cabinets may include a single speaker (typically 12" or 15"), multiple speakers of the same type (common formats include 2x10", 4x10" and 8x10"). Some bass cabinets use multiple different-sized speakers, such as a mixture of 12" and 15" speakers. In rare cases, some large bass cabinets (known as "bass bins") incorporate folded horns to boost the bass response. Bass amp cabinets may have a horn or tweeter built into a speaker cabinet which contains one or more woofer drivers. When a cabinet includes such a horn, the enclosure may also have an attenuator knob, for turning the horn volume up or down. Some more expensive bass cabinets with a woofer/horn approach may have additional features, such as circuitry to protect the speakers or horn from overloads or a biamplification option. Biamplification enables a bassist to use a crossover circuit to split their bass signal into two: a low frequency signal and