Why IC need not be parallel to each other?what does this mean?

Dear Student,
Indifference curves need not be parallel because the rate of diminishing marginal substitution of two goods is not necessarily equal to the other indifference curves.

In the above diagram the marginal rate substitution of I1 is different from I2.


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Dear Student,

 If you have an indifference curve for perfect substitutes they are parallel. But this is the only time. An indifference curve that has a 'curved' shaped (as opposed to the straight lines for perfect substitutes) are not parallel because they get closer to each other at the extreme points, yet they never cross. Two lines that are parallel are the same distance from each other at all points.
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