The name originates from the original implementation which was created by playing the same recording on two synchronized tape recorders, and then mixing the signals together. As long as the machines were synchronized, the mix would sound more-or-less normal, but if the operator placed his finger on the flange (that is to say, the rim) of one of the tape reels, that machine would slow down.
This would cause certain frequencies to become out-of-phase, thus cancelling, and other frequencies to be in phase, creating reinforcement. If the frequency response is plotted on a graph, the result resembles a comb, and so is called a comb filter. Once the operator took his finger off, the player would speed up until its tachometer was back in phase with the master, and as this happened, the frequency at which the peaks and dips in the comb filter occurred would change, producing a phasing effect. This phasing up-and-down the register can be performed rhythmically.
Creating the flanging effect with analog tape recorders is commonly called tape flanging. One characteristic of tape flanging which is difficult to reproduce otherwise is the thru-zero effect.
In the 1970s, advances in solid state electronics made the flanging effect possible using integrated circuit technology. Solid state flanging devices fall under two categories, analog and digital. The flanging effect in most newer digital flangers relies on DSP technology. Flanging can also be accomplished using computer software.