In an examination of Spanish comparisons of the types (1) 'cantor como un ruiseñor' (to sing like a nightingale) and (2) 'estar borracho como una cabra' (to be drunk as a she-goat), which are termed elative comparisons as they serve to denote a conventional specification of the highest or lowest degree of a gradable quality, it is shown that the properties of stability and idiomaticity, or semantic opaqueness, considered to be essential to all types of phraseological units, appear in elative comparisons as relative values only. In (1) the second branch of the comparison, traditionally the tertium comparationis, is compatible with the actions or states expressed in the first branch; whereas in (2) the meanings of the two branches are incompatible or obscurely compatible, invoking a metaphorization of the denotatum of the second branch (e.g., goats as a metaphor for drunkenness). Distributional, structural, and semantic features of elative comparisons are itemized with emphasis on the productivity of the elative system. (J. Hitchcock in LLBA 2003, vol. 37, n. 3)