Another obvious means of deploying microphones in the early days of stereo was to place three microphones spaced apart at the front of the orchestra, much more distant from each other than in the above systems. If only two microphones are used spaced apart by perhaps as much as two meters or more, what happens on playback is that the sound seems to cluster around the loudspeakers and there is a hole in the middle of the sound image. To prevent this, a centre microphone can be mixed in at a lower level so that the hole is filled. There is no theory on earth to explain why this works - being so dissimilar to the human hearing system - but it can work very well. The main drawback is that a recording made in such a way sounds terrible when played in mono.