Joint tenancy is a way for two or more people to co-own real estate. You and your fellow tenants can buy property together or the original owner -- your spouse or parent, say -- can add you to the deed. When one owner dies, her share passes to the other owners, and so does the responsibility for the mortgage.
Right of Survivorship
The right of survivorship makes joint tenancy different from ordinary co-ownership. When a property owner dies, his real estate usually passes according to his will, or state law if he dies without a will. As a joint tenant, you skip all that. You take ownership of his share -- or part of his share, if there are multiple co-owners -- regardless of what his will says. You take the added share without having to wait for the property to pass through probate.
During Life
While you and your joint tenants are all alive, you can divide up mortgage payments however you choose. If your co-ownership agreement says you must contribute a third or a half or whatever fraction of the mortgage, that's what you have to do. If your spouse puts you on the deed but doesn't ask you to help pay the mortgage, that's fine too. It's something everyone involved has to work out among themselves.
After Death
If, say, you and your partner are joint tenants and she dies, you inherit her half of the property. Whether she made half or all of the mortgage payments doesn't affect your situation: you're now the sole owner, so you're the one the bank wants a monthly check from. If you're one of several joint tenants, potentially you're all obligated to pay some of the mortgage. If you've already covered the situation in the ownership agreement, the agreement should be your guide.
Complications
In theory, your co-owner is free to put up her own property share as collateral for a loan, even without your consent. In practice, most lenders want all joint tenants to sign off on a loan. If your fellow tenant does find a lender, you could end up obligated to pay it back after her death. The lender can go to court and compel the sale of the entire house to get the debt paid off.
