During the earliest attempts to enhance heat transfer, plain (or smooth) surfaced were used. This surface requires a special surface geometry able to provide higher {displaystyle {hA}} {displaystyle {hA}} values per unit surface area in comparison with a plain surface. The ratio of {displaystyle {hA}} {displaystyle {hA}} of an enhanced heat transfer surface to the plain surface is called Enhancement Ratio " {displaystyle E_{h}} {displaystyle E_{h}}". Thus