A crossover is a device designed to divide audio information into smaller frequency ranges to comply with the requirements of different transducers in an audio reproduction system. This is accomplished by running the audio through a set of filters. For example, a two-way crossover may be comprised of a low pass and high pass filter where the low pass filter passes a signal with frequencies more suitable for a woofer and the high pass filter passes frequencies the tweeter can deal with. Crossovers can be passive or active designs. Passive crossovers are usually found inside speaker cabinets along with the speaker components. These often connect to the outside world via a single jack, but sometimes each speaker component also has its own jack in case one wants to bypass the built in passive crossover. Active crossovers are placed before the power amp. In that application each frequency range is given its own power amp and its own drivers. This is where the phrase bi-amping and tri-amping come from. There are a number of different types of filter configurations used in crossovers and they each produce subtly different results. One of the big variables is how steep the roll off is at the cutoff frequency. Common configurations are 12 dB per octave, 18 dB per octave, and 24 dB per octave. Each design has its own strengths and weaknesses, but in general steeper roll offs are considered better in modern applications.