Why do standard low resistance unit have four terminals?

Resistors of extremely high precision are manufactured for calibration and laboratory use. They may have four terminals, using one pair to carry an operating current and the other pair to measure the voltage drop; this eliminates errors caused by voltage drops across the lead resistances, because no charge flows through voltage sensing leads. It is important in small value resistors (100-0.0001 ohm) where lead resistance is significant or even comparable with respect to resistance standard value