The first 802.11 protocol used a spectrum in the 2 GHz and 5 GHz ISM region and evenly spaced channels roughly 20 MHz apart from each other. The channel bandwidth was 20 MHz, but later amendments from IEEE allowed 5 MHz and 10 MHz to operate as well. In the United States, 802.11b and g allow for eleven channels (other countries may support up to fourteen). The following figure depicts the channel separation. Three of the channels are non-overlapping (1,6,11):

802.11 specifies a spectral mask, which defines the permitted power distribution across each channel. The spectral mask requires the signal be attenuated to certain levels (from its peak amplitude) at specified frequency offsets. That said, signals will tend to radiate into adjacent channels. 802.11b using Direct Sequence Spread Spectrum (DSSS) has a completely different spectral mask than 802.11n using Orthogonal Frequency Divisional Multiplexing (OFDM). OFDM has a much denser spectral efficiency and therefore also sustains much higher bandwidth. Shown below are the channel and modulation differences between 802.11 b, g and n. Channel width limits the number of simultaneous channels from 4 to 3 to 1. The shape of the signal also varies between DSSS and OFDM. OFDM being much denser and thus capable of higher bandwidth.