A type of asteroid included in older classification schemes, where the ‘U’ stood for unclassifiable, for asteroids which could not be allocated to one of the five compositional types C, S, M, E, and R. However, more recent schemes group the asteroids into 9, 11, or 14 different types, depending on the technique used for analysis. In the 14-type scheme (the Tholen classification) the original U type has been subdivided into types R, A, M, P, Q, E, V, and T, in addition to the C, G, B, F, D, and S types used. In the new scheme, the letter U is appended to the classification of objects that possess an unusual spectrum for their type, e.g. CU, MU.