An efficient crushing and screening plant almost invariably employs the principle of stage reduction, with grizzlies, trommels, shaking or vibrating screens interspersed between stages to separate out the material that is already fine enough to by-pass the next step. The grizzly—stationary, rotary, or reciprocating—is suitable for the first stage, but thereafter the trommel or screen is more positive in its separation. Few trommels are used, except those on dredges and in sand and gravel plants.