The Monopolist's Marginal Revenue Curve versus Demand Curve

Selling one more unit does two things to revenue. The extra unit sells at the new, lower price, which adds revenue. But every unit that would have sold at the old higher price now sells for less, which subtracts revenue. The second effect means the revenue from one more unit is always less than its price, so the marginal revenue curve lies below the demand curve.

For a straight-line demand curve the arithmetic is tidy. Marginal revenue starts at the same point on the price axis, and falls twice as steeply, so it reaches zero at exactly half the quantity where demand reaches zero. The slider changes how steeply demand falls, and marginal revenue keeps hitting the axis halfway along.