Mortgage under line of credit

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Do you know you can have your mortgage under the secured line of credit?

When the mortgage rates started to drop in 2008, we unlocked our mortgage and put it in the secured line of credit we have at TD bank. At the time the line credit interest rate was prime rate. But in 2009 November, TD bank increased the interest rates on secured line credit to prime +1 and we went to the branch to talk the options and we were told that we can have a mortgage under the line of credit. We chose one year fixed rate at 2.4%. We just renewed it again for another year at 2.45%.

For example, if the line of credit is 200K and if the mortgage is 150K then you can use the balance 50K as line credit.