I know that there are avenues for deducting mortgage interest against US Federal income tax, but what about interest paid on a second mortgage?

For comparison, I receive a 1098 form from the lender showing my mortgage interest paid for the year. Under IRS rules I can deduct this. Under what circumstances, if any, would I be able to deduct interest from a second mortgage?

(I don't have a second mortgage. I saw a Twitter discussion of the "which debt to pay off first" variety and a second mortgage was one of the debts.)