The owner of an immovable property may mortgage his property to others for securing the repayment of the loans advanced or to be advanced. Doctrine of Tacking is a special doctrine which arises in certain type of transaction during the mortgage.
In this doctrine, the owner of an immovable property can mortgage his property to other for securing the different advances made. Here the same property is mortgaged again and again.
For example A may mortgage his immovable property with B for a loan. He can mortgage the same property to other people for loans from such persons. He can again mortgage the same property subsequently with B for a fresh advancement of money. Here the rule of priority in case of realisation rests with B; but only for the first advancement he made. He can claim the second advancement after the claims of other persons.
In the case of India the doctrine is dealt with under Section 79 of the Transfer of Property Act. As per the Indian law, if a mortgage is created to secure future and present advances of money and the maximum is limited, a subsequent mortgagee of the same property shall be postponed to the prior mortgagee in respect of all advances not exceeding the maximum limit if the subsequent mortgagee knows about the prior mortgage.
For example if A creates a mortgage of his property to B for securing a future loan of fifty thousand rupees and he borrows only half of the amount and thereafter A goes to C and mortgages the same property. Here C knows about the prior mortgage. Later A goes and borrows the other half amount of money from B. Later at the time of realisation C cannot say that B should only claim the first half and his other half can only be settled after the advance made by C.