The average variable cost (AVC), and average total cost (ATC) curves are both U-shaped. -The marginal cost (MC) curve shows how total cost changes when output increases by one unit. If the MC curve lies below the AVC curve, AVC is falling. Diminishing marginal returns means that eventually the MC curve slopes upward. At some point the MC curve lies above the AVC curve, and the AVC curve is upward sloping. -ATC is the sum of average fixed cost (AFC) and AVC. Initially the ATC curve falls as the quantity produced increases because the AFC is initially quite large, but drops rapidly as total fixed costs are spread over greater levels of output. However, eventually diminishing returns cause marginal product to fall below average product, and average product decreases. As a result AVC increases as the quantity produced increases. If AVC rises more quickly than AFC falls, then the ATC curve is upward sloping.