Coal size designation, used for anthracite only. Buckwheat is divided into four sizes: No. 1, or buckwheat; No. 2, or rice; No. 3, or barley; No. 4, or barley No. 2 or silt (sometimes also called culm or slush). Buckwheat No. 1 passes through a 1/2-in (12.7- mm) woven wire screen and over a 5/16-in (7.9-mm) woven wire screen, and through a 9/16-in (14.3-mm) round punched plate and over a 3/8-in (9.5-mm) round punched plate. 434 The American Institute of Mechanical Engineers has recommended that buckwheat No. 1 shall pass through 9/16-in holes and over 5/16-in holes, a screen with circular holes being used.