Final answer:
In a perfectly competitive market, the horizontal sum of all individual firms' supply curves forms the market supply curve, which indicates the total quantity supplied at various price levels.
Step-by-step explanation:
In a perfectly competitive market, the horizontal sum of all the individual firms' supply curves represents the market supply curve. This is because each firm is a price taker and can sell as much of the good as it wishes at the going market price. Therefore, at any given price level, the total quantity supplied by the market is the sum of the quantities supplied by all the individual firms. To obtain the market supply curve, you simply add up the quantities supplied by all the firms at each price level, which can be visually represented as horizontally summing their supply curves.
This coherent aggregation reflects the fact that in a perfectly competitive market, firms are reacting to the same price signals and independently adjusting their output, and as a result, the market's overall supply behavior can be described by combining their individual supply decisions.