To study customer maximisation manners or customer's equilibrium the acquaintance of price earnings line or budget line in essential. An indifference map points out choices of the customers only. But his actual assortment of the two commodities is based on his earnings and their prices. According to Prof. Salvatore, "The budget line presents all the diverse permutations of the two commodities that a customer can purchase, give his or her earnings and the price of the two commodities.

It is clear as to how many units of the two commodities A and B the customer can purchase depends upon his budget and the price of both the commodities. Now let us see how a customer's budget line will be affected by vary in price and earnings.

If the customer's earnings does not vary but the prices of commodities A and B vary, the inclination of his budget line will vary. The prices of A and B staying invariable, if the customer's earnings or budget increase or decrease his earnings or budget line will also vary. If earnings enhances the budget line will budge outwards. Either things can occur, there might be a parallel inward or outward shift of the budget line.

A customer is in equilibrium when specified his likings and prices of two commodities, he spends a given money earnings on the purchase of the two commodities in such a way as to get the maximum contentment. According to Koulsayaiannis, "The customer is in equilibrium when he maximises his utility given his earnings and the market prices."

1. The budget line ought to be Tangent to the indifference curve. Based on the postulations the customer can buy 5 units of A by spending the entire amount $10 on commodity A or on 10 units of B. The tablet illustrates some of the possible permutations on which $10 can be allocated.

Permutation	Commodity A in units	Commodity B in units
X	5	0
Y	4	2
Z	3	4
M	2.5	5
N	1.5	7
O	1	8
P	0	10

These gives us probable permutations specified by parameters X,Y,Z,M,N,O and P. The line PX shows the permutations of commodities A and B, given their prices, when he expends his earnings on them. Thus PX is the budget line. This budget equation is the equation of the line connecting the points X and P where X = I/Pa and P = I/Pb. The customer would choice permutations O and Y on a lower indifference curve I, because permutations Z and N is also available to him on a higher indifference curve I2. But here is another permutation M, which is on the highest indifference curve I3 on this budget line XP. Since all other permutations lie on lower indifference curves, they represent lower levels of contentment than permutations M which is the customer's equilibrium point. We may thus catalogue the conditions M which is the customer's equilibrium point. The customer is in equilibrium when his budget line is tangent to an indifference curve. XP is the tangent curve I3 at M. Let us construct how the curve looks with the above parameters.

In the above study, of the customer's equilibrium it was alleged that the earnings of the customer stays invariable, given the prices of the commodities A and B. Given the likings and choices of the customer and the prices of the two commodities, if the earnings of the customer varies, the effect it will have on his purchase is known as the earnings effect. If the earnings of the customer enhances his budget line will budge upward to the right, parallel to the original budget line. On the contrary, a fall in his earnings will budge the budget line inward to the left. The budget lines are parallel to each other because relative prices stay unvaried.

The substitution effect relates to the diverge in quantity demanded resulting from a diverge in the price of good due to the substitution of relatively cheaper good for a dearer one, while keeping the price of the other good and real earnings and likings of the customer as invariable.

Slutsky explained the substitution effect by taking the apparent real earnings of the customers constant. With the drop in the price of good A when the real earnings of the customer improves, it is in synch in such a way that the customer could purchase the same package of the two commodities as prior to the price diverge if he liked, so that his real earnings stays invariable.

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