These reel to reel decks are usually analogue sound devices using a magnetic tape of ¼ inch (i.e. 6.25 cm) width. This plastic tape is covered by a special magnetic substance which is being made into magnetic domains when it is being recorded. The tape recorder must have a record, playback and an erase head which transfer the signal onto the tape. The more expensive tape decks have three or six separate heads. Tape recorders combine knowledge both on mechanics and electronics. There must be very precise motors and moving parts, but also a complicated circuit board. It has to contain transistors, amplifiers etc.