A load coil is an inductor placed on the local loop by the phone company to improve voice quality. Load coils improve low frequency signals (voice) but supress high frequency signals (DSL). They are typically placed on the lines starting at 3000 feet in 6000 feet intervals and are designed to suppress the exact signal frequencies at which DSL operates.

Load coils improve low frequency signals (voice) but suppress high frequency signals (DSL). The effect, from the perspective of the DSL equipment, is the equivalent to adding 20,000 feet to the line length. There are supposedly newer load coils that are more DSL friendly but typically DSL does not operate properly through load coils, and they are a major source of loop disqualification.