What is the extra, 5th, pin on micro usb 2.0 adapters for?



Here is an image with the different connectors.  Most of them have 5 pins, but the A-type host only has four.


(source: wikimedia.org)  

It's for On-The-Go, to select which device is the host or slave:


  The OTG cable has a micro-A plug on one side, and a micro-B plug on
  the other (it cannot have two plugs of the same type). OTG adds a
  fifth pin to the standard USB connector, called the ID-pin; the
  micro-A plug has the ID pin grounded, while the ID in the micro-B plug
  is floating. The device that has a micro-A plugged in becomes an OTG
  A-device, and the one that has micro-B plugged becomes a B-device. The
  type of the plug inserted is detected by the state of the pin ID .



