An autotransformer that has its winding divided into a number of equal-turn sections (usually 10) so that when an alternating voltage V is applied to the whole winding, the voltage across each section is nominally V/10. The progressive voltages from one end to the section junctions are thus V/10, 2V/10, 3V/10, … , and 9V/10. This voltage division can be realized with errors considerably less than 1 part per million of V, and such units therefore find wide use as standards of ac voltage ratio in the discipline of electrical measurements.