The need for calculating an arc elasticity arises when a measure of sensitivity to change involving two discrete points on a relationship is desired. Unfortunately, numerous, non-unique representations are possible because there is no unique reference point from which to calculate the measure. The matter of non-uniqueness was addressed in an article in this journal by Morrill [6] in which various representations of a specific arc elasticity measure?the arc elasticity of demand?are discussed. In most economics textbooks, the dominant arc elasticity measure used is the definition derived by Allen [1]. It is equivalent to drawing a linear demand curve through the two points and calculating the point elasticity at the midpoint. One could also view this as using the average for the reference base in calculating percentage changes in quantity and price. Allen's definition has not gone unchallenged. Its earliest critic was Lerner [4,5], who derived and proposed an alternative measure. Stigler [9] has offered, in addition to the arc elasticity, two alternative approximations, namely, a total revenue test, and fitting a curve through the data points. Most recently Seldon [8] and Vaughn [10] have rekindled the controversy by restating and refocusing previous work on biases in the Allen definition. Through it all, however, Allen's measure continues to prevail as the standard fare.1 In this article, we illustrate an additional problem with Allen's definition of the arc elasticity: there is a finite feasible set for magnitudes of the percentage changes. In addition, we illustrate that the inherent bias identified by Vaughn [10] fundamentally de pends upon the presence of a feasible set. Finally, we conclude by recommending an alternative, log-based elasticity definition to supplement the conventional Allen formula. We believe this definition has desirable properties and merits consideration by authors and instruc tors for undergraduate courses in economics, including principles.