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They don’t overlap the way other channels do. Think of the channels as fm radio stations. If you wanted to listen to 106.5, for example, you would still probably pick up a signal at 106.4 or 100.6. The signal is so narrow as to limit itself precisely to the channel. WiFi is the same way, and that bleed over can affect nearby channels, so that there are multiple signals overlapping, which can degrade performance. 1, 6, and 11 don't overlap to the same extent that the other channels do. For example, channel 3 would have signal overlap with 1, 2, 4, 5, and 6, where as channel 1 would have overlap with only 2, 3 and possibly 4.This chart illustrates it, and might make it more clear. http://www.geekzone.co.nz/inquisitor/2996