A ﬁre line is a break in fuel continuity. It can be a natural barrier, such as a river or cliff, but in wildﬁre containment, ﬁre lines are more commonly human constructions. A rule of thumb holds that the width of a ﬁre line on reasonably level ground should be one and a half times the height of the ﬂames to be contained, and considerably wider on steep slopes. Fire lines dug down to mineral soil before an oncoming ﬁre are known as hot lines. Burned-out areas are known as cold trails. Lines put down by human labor are hand lines, and preliminary lines are scratch lines. Fire lines constructed by use of equipment such as bulldozers are known as cat lines, tractor lines, or plow lines. Lines involving chemicals, foam, or water are called wet lines or retardant lines. Those put down to contain backﬁring, the burning of fuels ahead of the main ﬁre in hope of replacing an uncontrollable ﬁre with one that’s more likely to be controlled, are known as back lines.