Passive crossovers are located between the amplifier and the drivers, and for this reason, are sometimes referred to as high level networks. These networks are comprised of inductors, capacitors, or some combination of each. The basic characteristics of both types is the same. As the frequency passing through an inductor increases, so does the inductive reactance of the coil. This rise in impedance at higher frequencies allows us to use the inductor as a filter that passes low frequencies but chokes off high ones.
Capacitors do just the opposite.
As frequency decreases, the capacitor's reactance increases. That is, capacitors pass high frequencies and filter out low ones. These capacitors are generally non-polarized electrolytics, polypropylene, or Mylar and consist of interleaved layers of foil and insulating material. While there is some insertion loss, the filtering action actually takes place due to the impedance mismatch created by the network. As frequency goes down, the capacitive reactance of the capacitor goes up.