The Appalachian Trail is marked for daylight travel in both directions, using a system of paint "blazes" on trees, posts, and rocks. There are some local variations, but most hikers grasp the system quickly. Above treeline, and where snow or fog may obscure paint marks, posts and rock piles called "cairns" are used to identify the route.
What is a "blaze"?
A blaze is a rectangle of paint in a prominent place along a trail. White-paint blazes two inches wide and six inches high mark the A.T. itself. Side trails and shelter trails use blue blazes; blazes of other colors and shapes mark other intersecting trails. Two white blazes, one above the other, signal an obscure turn, route change, incoming side trail, or other situation that requires you to be especially alert to changes in direction. In some areas, one of the two blazes will be offset in the direction of the turn.
White-paint blazes mark the A.T., and may be located on trees, rocks, or posts.
Double blazes, sometimes offset, signal an obscure turn, route change, or incoming side trail.
Rock cairns identify the route above treeline and where snow and fog may obscure painted blazes.
What if I don't see blazes?
Distance between blazes varies. In some areas, blazes are almost always within sight; in areas managed as wilderness you may encounter only four or five per mile. If you have gone a quarter-mile without seeing a blaze, stop. Retrace your steps until you locate a blaze. Then, check to make sure you haven't missed a turn. Often a glance backwards will reveal blazes meant for hikers traveling in the opposite direction. Volunteer trail maintainers regularly relocate small sections of the path around hazards or undesirable features or off private property. When your map or guidebook indicates one route, and the blazes show another, follow the blazes.