law: Indian Contract Act, 1872 
  29. Agreements void for uncertainty
  Agreements, the meaning of which is not certain, or capable of
  
  being made certain, are void.
   Illustrations 
  (a) A agrees to sell B "a hundred tons of oil". There
  
  is nothing whatever to show what kind of oil was intended. The agreement is
  
  void for uncertainty.
  (b) A agrees to sell B 100 tons of oil of a specified
  
  description, known as an article of commerce. There is no uncertainty there not
  
  make the agreement void.
  (c) A who is a dealer in coconut-oil only, agrees to sell to B
  
  "100 tons of oil". The nature of A's trade affords an indication of
  
  the meaning of the words, and A has entered into contract for the sale of one
  
  hundred tons of coconut-oil.
  (d) A agrees to sell to B "all the grain in my granary at
  
  Ramnagar". There is no uncertainty here make the agreement void.
  (e) A agrees to sell to B "1000 maunds of rice at a price
  
  to be fixed by C". As the price capable of being made certain, there is no
  
  uncertainty here to make the agreement void.
  (f) A agrees to sell to B "my white horse for rupees five
  
  hundred or rupees one thousand". There is nothing to show which of the two
  
  prices was to be given. The agreement is void.