Rowing exercises must be broadly based to achieve these various performance goals. The development of technical rowing skills will be done primarily upon the water. A solo sculler or a crew can practice such aspects of a competition as the start, changes in cadence, and general unified strokes. Crews will sometimes use a large indoor swimming pool as a simulator, with the boat tethered, as a practice facility. Rowers can also use a stationary training device known as an ergometric rowing machine to simulate the resistance encountered in the rowing motion; these machines have the advantage of having a variable resistance.