Match heads contain phosphorus, potassium chlorate and sulfur, and when heated by friction, the phosphorus ignites, causing the other two materials to burn. When ignited, the potassium chlorate produces oxygen in amounts far exceeding what is typically found in the surrounding air. The oxygen and sulfur mixture burns steadily, igniting the matchstick to produce a usable flame.
In a strike-anywhere match, the match head contains all the chemicals needed to trigger the reaction and fire. The phosphorus, potassium chlorate and sulfur are mixed throughout the head of the match, and crushed glass is added to provide extra friction. This makes it possible to ignite a strike-anywhere match on any rough surface, but it does present a safety risk. Unprotected matches may rub against one another or rough surfaces, causing an unexpected ignition.
Safety matches prevent this problem by removing the phosphorus from the match head, instead using it to impregnate the rough strip along the back of the matchbook. Without the phosphorus, friction alone is not enough to start the reaction and ignite the match. The match head must be dragged against the strip embedded with phosphorus to provide the final piece of the chemical puzzle.