What is Doctrine of Tacking?

Doctrine of tacking is explained in section 93 of the Transfer of Property Act. The section prohibits tacking. When a subsequent mortgagee(puisne) discharges the legal mortgagees(first mortgagee) claim, he subrogates himself to the first mortgagees claim on such mortgaged property. This is called tacking. But this is prohibited under section 93 of the Act. Even after puisne mortgagee acquires the first mortgages claim by discharging the first mortgagee’s dues, he has priority to the claim acquired which the first mortgagee had and must stand and wait in his proper place for his (as a puisne mortgagee) claim.
However an exception is provided in section 79 where tacking is allowed.

Ex: Mr. A mortgages his property to Y bank to secure the repayment of borrowed amount of Rs. One lakh. Later on Mr. A mortgages the same property to C to secure the repayment of loan of rupees one lakh. C notifies the second mortgage to Y Bank, and on the date of second mortgagee, amount due to the Y Bank was Rs. fifty thousand. Y bank though being aware of second mortgage lends to A making the total amount due exceeding one lakh. Y bank is entitled and has charge over the mortgaged property upto Rs. one lakh and has priority over C.