Units of one commodity which a consumer is willing to sacrifice for one extra unit of another commodity to maintain the same level of satisfaction, is known as marginal rate of substitution. It can be measured by the ratio between change in units of one commodity to change in units of another commodity. (MRSxy = ). MRS represent the slope of indifference curve. As we move downwards along the same indifference curve its slope i.e. marginal rate of substitution continuously diminishes. MRS continuously diminishes because as the stock of one commodity increases its marginal importance for the consumer will continuously decreases and as the stock of another commodity decreases its marginal importance for the consumer will continuously increases. Therefore consumer is ready to sacrifice lesser and lesser units of one commodity for the every extra unit of another commodity to maintain the same level of satisfaction. Therefore MRS continously diminishes and due to that reason indifference curve becomes convex towards the origin and rectangular hyperbola in shape.