The Budget line and its economic interpretation
The indifference curve shows us consumer preferences but it does not show us the situation in the market place. Here the consumer is constrained by income and by the prices of X and Y. They can both be shown by a budget line. Suppose that product X costs K£2 per unit and product Y K£1 per unit and that the consumer's income is K£10.
A budget line shows all the combinations of two products which can be purchased with a given level of income. The slope of the line shows the relative prices of the two commodities.
If the consumer is inside the budget line, e.g. at point E he is consuming les than the income. Thus he can consume more of X or more of Y or more of both. If he is on the budget line e.g. at point C he is spending the full budget. He is said to be consuming to budget constraint. To consume more of X e.g. moving from C to D, he must consume less of Y and vice versa. For a given budget and given price, he cannot be at a point off the budget line to the right, e.g. at point F.