A three-phase system has three, accurately synchronized single-phase systems used together. Each of these three single-phase sine waves has the same amplitude as the others but there is a phase displacement of exactly one-third of a cycle between these voltage waves. This symmetric arrangement is called a "balanced set of voltages." Since each phase winding has two wires, this would be a six-wire system. However, it is easier to connect three of these wires (one from each phase) to a common wire shared by all three phases. This forms a four-wire system where this common wire is called the "neutral." Line-to-neutral voltages are measured from the neutral to any one of the other lines. The line voltage (line-to-line), is measured between any two of the other lines. This system is called a "wye" wiring system.
An alternative to the wye configuration is where the three balanced single-phase 
windings are interconnected in a "delta" configuration, with each coil
connected to both of its neighbors. This produces a three-wire, three-phase system.
Here, the line voltage is measured between any two of the lines.
Power can be transmitted in a three-phase system over either a three-wire or a four-wire system. Some differences will be pointed out in later sections.
