I got my first stepper running with Arduino. I found an old pair of steppers in one of the boxes where I keep such things for years now, (»maybe one needs them one day«).
This is the stepper motor pair I have:
It is a 6 wire unipolar motor: It seems as if bipolar steppers have 4 wires, unipolar ones 5 or 6 wires.
Now I had to find out what are the common ground wires. Here I found
...one way to distinguish common wire from a coil-end wire is by measuring the resistance. Resistance between common wire and coil-end wire is always half of what it is between coil-end and coil-end wires. This is due to the fact that there is actually twice the length of coil between the ends and only half from center (common wire) to the end.
Testing the resistance with an ohmmeter the two center wires could be identified as the common wires.
Unfortunatelly, I only had a L293D step motor driver chip which is meant for bipolar motors. But as it turns out, this is not a problem when you connect the 2 common wires
The MotorKnob tutorial on arduino.cc (big picture of the breadboard layout) shows how to connect a bipolar motor to the Arduino. When you don't connect the common wire, the unipolar motor works with the L293D driver.