Each side of the fire is described in terms of head, flank, and rear. The head is the fastest spreading part of a fire's perimeter. The head is usually the side toward which the wind is blowing, and will also often be the upslope side of a fire. The head of the fire is of primary interest.
The right and left flanks describe the sides of the fire. Flanks are perpendicular to the head of the fire.
The rear of the fire is the side of the fire opposite the head.
Example 1 - Plot the perimeter and label the head, rear, and flanks of a fire with ROS of 4 chains/hour and PT of 3 hours. Assume the wind is blowing from left to right across the page.
The head of the fire on your drawing should correspond to the edge of the fire's forward perimeter at the projection time. The distance from head to rear will correspond to the spread distance, in this case 12 chains.