A temper line also known as a Hammond, line is a line on the knife blade that is created by differential heat treating the steel. When you differentially heat treat the steel you harden only the edge of the steel as opposed to hardening the entire knife. After hardening the edge only and then tempering the edge, there is a point between the hard steel and the softer steel that shows the transition between the two. This line can be seen if desired by placing the blade in an acid solution for a few seconds. The acid attacks the steel and begins to eat the steel away. Since the metal is both hard and soft it attacks it at different rates. After just a few seconds in the acid, the blade is removed and lightly sanded. The result is a very nice looking blade with a very obvious line showing the transition between the hard and soft steels. There is a knifemaker that I think has perfected this technique better than any other. This knifemaker is a full time maker and his knives are very high qu