Price of related goods is one of the factor or determinant which affects the Demand of a commodity keeping other factors like Own price of Commodity, Consumer's income, TASTE and preference , Expectations, other Cause etc. being Constant.

Related goods are of two types :
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=> Substitutes good
=> Complementary goods

[ 1 ] . Substitute goods
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Substitutes goods are those goods which Can be USED as in one place of other good .
Simply here one good replaces other or substitute other....
or One good can be used in place of other .
Examples of Substitute goods are :
=> Apples and BANANA
=> Tea and Coffee
=> Pepsi and RC Cola
etc.,
If price of one of the substitute good increses , the quantity demanded of other increses and vice versa...
For example , if price of apples rises , we purchase less commodity of apples and purchase more commodity of its substitute banana...as a result the demand of banaana rises...

[ 2 ] . Complementary goods
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Complemetary goods are the one which are being used together...or Both goods are complement of each other...
Examples are :
=> Car and Petrol
=> Bread and Butter
=> COMPUTER hardware and software
etc..,
If the price of one complementary good increases , the demand of the other increses too...If prices decreses , demand of the complement goods falls too...
For example , If price of butter increses , the demand for the bread falls and on the other end...if price of butter DECREASES , demand of its complementary good i.e bread decreases too...