Brigadoon - An Occasional Corner on the Internet
Popular 0.1" header with 2 pins, 3 pins, 4 pins, 6 pins, 8 pins and 10 pins are available for hobby use. This site usually uses the 2-pin, 3-pin, 4-pin and 8-pin versions in its projects. They are normally used inside projects, so the connections can be more flexible in general, but fixed within the project to present confusion.
0.1" Header with Crimp Pins
Designed by Mark Little
The following conventions are used for the plugs for Arduino I/O allocations, but may be altered depending on the project's requirement.

2-Pin Connector

Pin Description
1 Supply +5/12 Volts
2 Supply Earth

3-Pin Connector

Pin Description
1 Supply +12 Volts
2 Supply Earth
3 Supply -12 Volts

8-Pin Connector

Pin Description
1 Supply +5 Volts (See Note 3)
2 Supply/Signal Earth
3 Signal Line 1
4 Signal Line 2
5 Signal Line 3
6 Signal Line 4 (See Note 4)
7 Signal Line 5 (See Note 2)
8 Signal Line 6 (See Note 2)
Note 1: Unless specifically stated otherwise, the signal on each line should be uni-polar in the range 0 V to +5V.

Note 2: Pins 7 and 8 may be used for bi-polar RS-232 transmit and receiver.

Note 3: Pin 1 may be used for a signal line if the +5V power connection is not required.

Note 4: Pin 4 may be used a Signal Earth if required.

4-Pin Connector

Pin Description
1 Supply +5 Volts (See Note 3)
2 Supply/Signal Earth
3 Signal Line 1
4 Signal Line 2