From raspberrypi.org's GPIO page:
%%blockquote
A powerful feature of the [Raspberry Pi] is the row of __GPIO__ (general-purpose input/output) pins along the top edge of the board. A 40-pin GPIO header is found on all current Raspberry Pi boards (unpopulated on Pi Zero and Pi Zero W).
%%

The GPIO header is the main set of input/output connections on a Raspberry Pi, apart from power supply, USB, HDMI, camera, audio and network connections. It consists of two rows of twenty pins (forty pins in total) along the top of the Raspberry Pi circuit board:

[{Image src='attach/GPIO/gpio-rpa.jpg' caption='GPIO pins on a Raspberry Pi A+' align='center'}]


!! Pinout

The GPIO pins have two labelling schemes, one based on the physical pin location on the header; the other, called the BCM (Broadcom Pin Number), based on their usage. GPIO software libraries force you to select which pin numbering scheme to use (e.g., BCM pin 7 is physical pin 26, BCM pin 12 is physical pin 32).

Physical pin 1 is at lower left, pin 2 upper left, pin 39 at lower right, pin 40 at upper right. 

[{Image src='attach/GPIO/raspberry-pi-pinout.png' link='https://pinout.xyz/#' caption='The Raspberry Pi GPIO Pinout (courtesy pinout.xyz)' align='center'}]

[{Image src='attach/GPIO/pinout-cli.png' caption='typing "pinout" on the command line' align='right' class='imgFloatRight'}]

You can type:
{{{
  % pinout
}}}
in the Raspbian OS and it will display information about your GPIO usage.

!! Interrupts

On how to use GPIO interrupts on a Raspberry Pi.

* [Raspberry Pi GPIO Interrupts Tutorial|https://roboticsbackend.com/raspberry-pi-gpio-interrupts-tutorial/]
** [How to use interrupts with Python on the Raspberry Pi and RPi.GPIO|https://raspi.tv/2013/how-to-use-interrupts-with-python-on-the-raspberry-pi-and-rpi-gpio] 
** [How to use interrupts with Python on the Raspberry Pi and RPi.GPIO – part 2|https://raspi.tv/2013/how-to-use-interrupts-with-python-on-the-raspberry-pi-and-rpi-gpio-part-2] 
** [How to use interrupts with Python on the Raspberry Pi and RPi.GPIO – part 3|https://raspi.tv/2013/how-to-use-interrupts-with-python-on-the-raspberry-pi-and-rpi-gpio-part-3] 



!! Links

* [Interactive GPIO Pinout|https://pinout.xyz/#]
* [GPIO|https://www.raspberrypi.org/documentation/usage/gpio/] page on raspberrypi.org
* [Raspberry Pi GPIO Interrupts Tutorial|https://roboticsbackend.com/raspberry-pi-gpio-interrupts-tutorial/] from ''[The Robotics Back-End|https://roboticsbackend.com/]''

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[{Tag Hardware GPIO}]