Structure refers to the framework of a wine based on the sum of its main components: fruit, acid, tannin, alcohol, and sugar (if any). If all these elements are in proper proportion so that none overpower and unbalance the wine, then other, more complex aromas and flavors can reveal themselves. A good wine has a firm structure, while a wine that, for example, has little acidity and tastes flat or flabby is said to be lacking in structure. Wines that are meant to be aged for several years must be firmly structured; that is, they must have concentrated yet balanced levels of acidity, alcohol, fruit, and tannin to prevent them from becoming unstable or flat while they mature.