Primary and secondary channels
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Primary and secondary channels

• When channels are bonded together to create a wider channel-width for higher throughput, there is a Primary and secondary channel.
  
• The channels used are designated as primary and secondary and are indicated by two fields in the body of certain 802.11 management frames.
• A positive or negative offset indicates whether the secondary channel is one channel above or one channel below the primary channel.
  
  Example:
  36, +1 = Channel 36, 40 are bonded. Channel 36 is primary and channel 40 is secondary
  40, -1 = Channel 36, 40 are bonded. Channel 40 is primary and channel 36 is secondary
  

Primary channel

• If 20/ 40-capable stations transmit by using a single 20 MHz channel, they must transmit on the primary channel and not the secondary channel.
  
• APs use the primary channel to transmit beacon frames and to perform media access control tasks.
  

Secondary channel

• Used to forward Data frames
  

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.