Abstract: A fin strip of the kind used as secondary heat transfer surface in heat exchangers. Convolutions in the strip are formed to increase the developed length of a fin strip and thereby to increase the surface area which can be accommodated in a given space, particularly an annular space, without obstruction to flow. In a strip corrugated to provide alternating peaks and valleys, peaks are depressed to form spaced apart peak portions interconnected by intermediately positioning valley portions which add to the developed length of the strip without reducing the space between strip valleys. In an annular space occupied by a fin strip made in accordance with the invention fin density is substantially uniform throughout.