The moving-coil principle is by far the most widely used, as it can be implemented in very cheap transistor radio speakers, public address systems, and also top-quality studio monitors, plus all performance levels and applications in between. The ribbon loudspeaker principle has sometimes been employed in high-frequency applications (‘tweeters’) and has recently also been employed in large full-range models. Horn loading is a technique commonly employed in large public address loudspeaker systems. Numerous three-way loudspeaker systems have also appeared where a separate mid-range driver is incorporated along with additional crossover components to restrict the frequencies feeding it to the mid-range. In active systems, amplifiers can be better matched to loudspeakers, and the system can be designed as a whole, without the problems which arise when an unpredictable load is attached to a power amplifier. Loudspeaker positioning has a significant effect upon the performance.