This paper presents a new coupling metric (Coup), which is based on the formal definition of methods and variables of classes, and packages. The proposed metric has been validated theoretically against Briand properties as well as empirically using packages taken from two open source software systems and four experienced teams. We measure Coup value by our own CC tool. An attempt has also been made to present a strong correlation between Coup values and understandability of the packages and between Coup values and modified classes of the packages. The results indicate that Coup is used to predict understandability and modifiability of a package in Object-Oriented design. Finally this paper proves that Coup is a better predictor of understandability and modifiability of a package than other existing coupling metrics in the literature.