increase (maybe by only 1%, maybe by 40%, maybe by 70%). A superior good is a normal good for which the proportional consumption increase exceeds the proportional income increase. So, if income increases by 50% then consumption of a superior good will increase by more than 50% (maybe 51%, maybe 70%). In economics terminology, all goods with an income elasticity of demand greater than zero are "normal", but only the subset having income elasticity of demand > 1 are "superior". Some articles in the microeconomics discipline use the term superior-good as an alternative to an inferior good, thus making "superior goods" and "normal goods" synonymous. Where this is done, a product making up an increasing share of spending under income increases is often called an ultra-superior good. Art history Though often verging on the meaningless in modern marketing, "luxury" remains a legitimate and current technical term in art history for objects that are especially highly decorated to very high standards and use expensive materials. The term is especially used for medieval manuscripts to distinguish between practical working books for normal use, and fully illuminated manuscripts, that were often bound in treasure bindings with metalwork and jewels. These are often much larger, with less text on each page and many illustrations, and if liturgical texts were originally usually kept on the altar or sacristy rather any library that the church or monastery who owned them may have had. Secular luxury manuscripts were commissioned by the very wealthy and differed in the same ways from cheaper books. "Luxury" and luxury arts, may be used for other applied arts where both utilitarian and luxury versions of the same types of objects were made. This might cover metalwork, ceramics, glass, arms and armor, and a wide range of objects. It is much less used for objects from the fine arts, with no function beyond being an artwork: paintings, drawings, and sculpture, even though the disparity in cost between an expensive and cheap work may have been as large. Market Characteristics Luxury goods have high income elasticity of demand: as people become wealthier, they will buy proportionately more luxury goods. This also means, however, that should there be a decline in income, its demand will drop more than proportionately. The income elasticity of demand is not constant with respect to income and may change signs at different levels of income. That is to say, a