Normally, first mortgages are primary financing on the residence of the borrower. These can be variable or fixed rate loans.
Interest paid is normally deductible for personal income tax purposes.
Check with your tax advisor to determine deductibility. With a variable rate loan, the note term is shorter than the full amortization of the loan. For example, a 15 year mortgage with a note that renews annually. Payments are based on the full term of the loan. A fixed rate loan is usually a longer term than a variable rate.