Retailers may also qualify coffee labels by grade name. Grading is a device for controlling the quality of an agricultural commodity so that buyer and seller can do business without personally examining every lot sold. Coffee grading terminology is, unfortunately, varied and obscure. Every coffee-growing country has its own set of terms, and few are distinguished by logical clarity. Kenya AA is an exception: Clearly AA is better than A or B. But though the Colombian terms excelso and supremo are both laudatory, one could hardly determine by reason alone that supremo is the highest grade of Colombian coffee, and excelso a more comprehensive grade consisting of a mixture of supremo and the less desirable extra grade. Although we may be aware that altitude is a prime grading factor in Central American coffees, one could hardly guess without coaching that strictly hard bean refers to Guatemalan coffees grown at altitudes of 4,500 to 5,000 feet, and hard bean to those at 4,000 to 4,500 feet. The higher the altitude, the slower-maturing the bean, and the harder and denser its substance–hence hard bean.