Since the introduction of 802.11g each channel in the 2.4 GHz spectrum is 20 MHz wide. Therefore, in the US it is recommend to use channels 1, 6 and 11 to get maximum spectral use and to avoid deploying channels that will interfere with one another. To see why take a look a figure 1.
STA's with overlapping channels will not defer to each when transmitting and will therefore, cause CCI (co-channel interference) which results in high retries and possible channel saturation.
Bonding is a process where two channels are grouped together or bonded. Theoretically bonding can lead to higher throughput. However, bonding in 2.4 is never a good idea except in a completed isolated environment due to the CCI and ACI (adjacent channel interference), that will result.
So what should you do if your neighbor has used bonding in 2.4 GHz? Enable the 40 mHZ intolerant bit because when any station hears a Beacon frame with this bit set it must revert to using a 20 MHz wide channel .