He created a design with three index arms and verniers. The B-1 engine then shut down, followed by the sustainer and verniers. "Direct verniers" are the most common. It had a main fixed combustion chamber and four small verniers to supply vector control. The verniers provided roll control and final velocity trim. Direct and retrograde verniers are read in the same manner. From there on, the sustainer and verniers would operate by themselves. The verniers provided steering, eliminating the need for gimballing of the main engine. "Retrograde verniers" are found on some devices, including surveying instruments. Verniers allowed measurement to a fraction of a degree, such as with a turn-of-the-century transit. The sustainer and verniers continued operating until missile destruction.