20MHz channels at 2.4 GHz (Channels 1, 6, 11)
Now that I have a pair of Ubiquiti UAP Pro access points and an Airport Extreme, its time to look at the right frequency design given that all of these use huge 40MHz channels to get 300-450Mbps, but this diagram from wikipedia is instructive. It's worth noting that MacBooks artificially limit themselves to 20Mhz at 2.4GHz but use 40MHz at 5GHz and iPhones 4S and earlier are limited to 20MHz as well at 2.4GHz, but the new iPhone 5 supports 5GHz and 40MHz and single data stream (or 150Mbps!). So you want to design 20MHz at 2.4GHz if you have Mac and 40MHz at 5GHz and have enough 5GHz APs so MacBooks and iPhone 5s kick up there. Most of the time you want to use 1 and 11 but it depends on what else is radiating.
There are 14 channels designated in the 2.4 GHz range spaced 5 MHz apart (with the exception of a 12 MHz spacing before Channel 14). As the protocol requires 25 MHz of channel separation, adjacent channels overlap and will interfere with each other. Consequently, using only channels 1, 6, 11 is recommended in the US to avoid interference for 20MHz. In much of the world, the four channels 1, 5, 9, 13 are recommended. There are exceptions to this however, for example in the UK, where British Telecom recommend use of three channels 1, 6, 11. Using the 3-channel system is recommended, since many existing access points are on channel 6 by factory default, causing the channel 6 to be likely to be in use anyway which would mean you should set it to 1 and 11.
via List of WLAN channels - Wikipedia, the free encyclopedia.
40MHz in 2.4GHz range
This is difficult as 40MHz takes up 8 of the allowed 11 channels of 5MHz each in the US and many clients don't support it and you are vulnerable to interference too.
40MHz in 5GHz range
In the 5GHz range, there is much more spectrum as these channels are already 20MHz, so the allowed channels are, where the +1 and -1 means you take a channel above or below as Intel says. So if you have some laptops, then you want to push them all to 2.4GHz 40MHz channel at channel 3 or at 5GHz and let the iPhones live in the 20MHz down low at channel 11.
Supported combinations of 802.11n channels at 5GHz with 40MHz bandwidth are:
(36,1) (40,-1); (44,1) (48,-1); (52,1) (56,-1); (60,1) (64,-1); (100,1) (104,-1); (108,1) (112,-1); (116,1) (120,-1); (124,1) (128,-1); (132,1) (136,-1); (149,1) (153,-1); (157,1) (161,-1)