Bipolar stepper motors have only a single winding per phase. Four wires are connected to the motor windings, and there is no center tap to VM.

Figure 2-2 Bipolar Stepper Motor

Bipolar steppers require more complicated driver electronics (typically an H-bridge for each phase) to be able to reverse the current in the winding. However, bipolar drivers use the full amount of copper windings, so the motor delivers more torque.