(MONEY Magazine) – One of the most confusing aspects of the mortgage process is the question of when the interest rate you'll pay is locked in. The problem has been particularly acute in recent months: Many homeowners who applied for loans in January, when mortgage rates bottomed at 8.31%, found they had to pay rates as much as three-quarters of a point higher when the time came to close. If you apply for a mortgage today, will you get today's rate? Not necessarily. It's the policy of some lenders to set your rate on the date when you apply. But others use the rate that's prevailing when your loan is approved; when the bank issues a formal loan commitment; or when the closing takes place, by which time rates may have moved sharply up or down. If your lender is in the latter group and you want to protect yourself against a sudden spike in rates, you may be able to lock in your rate up front, usually for a fee. You'll pay anything from a few hundred dollars to 1% of the loan amount, though many lenders will credit the fee toward other costs at closing. It makes no sense to pay a big, nonrefundable lock-in fee unless the lender offers an exceptionally low rate. Be aware, though, that some lock-in deals are better than others. If mortgage rates in general fall during the lock-in period, some lenders will adjust your rate downward, but others will stick you with the rate that you locked in. Most lock-ins guarantee the rate for 45 days. But if the lender has a big backlog of loan applications, your lock-in may expire before you can go to closing. To guard against that problem, deal only with a lender who promises to extend the lock-in if your loan cannot be processed in time. ''Without a doubt, lock-ins are the most frequently misunderstood part of the mortgage process,'' says Michael Hoogendyk, executive vice president of the National Association of Mortgage Brokers. ''To protect themselves, consumers must ask the right questions and get everything in writing.'' -- B.K.

BOX: COMMON LOAN INDEXES

Prime rate 6.50% One-year Treasury constant maturity 4.63% 11th District cost of funds 5.80% National mortgage contract rate. 14%

Note: Prime rate is as of May 5. Others are averages for March, February and March, respectively. Source: HSH Associates. For the latest information on mortgage rates in your area, plus a kit that will help you select the right loan, call toll-free for MONEY Magazine's Mortgage Match at 800-243-8474. Cost: $29.95

CHART: NOT AVAILABLE CREDIT: Source: HSH Associates CAPTION: LEADING HOME-EQUITY LINES IN THE LARGEST METRO AREAS LEADING 30-YEAR FIXED-RATE MORTGAGES