Prices and basic ground rules are the same on all the seven giant lines. You buy a book of four coupons, each good for a one-way Coach trip, regardless of length or how many connections you might have to make. Books cost $600 to $700 depending on the airline. Check with your travel Agent to find the best airline that will fit for your travel needs. You exchange each coupon for a one-way trip within the lower 48 states plus nearby points in Canada and the close Atlantic/Caribbean islands with THAT airline. You can fly to Hawaii or Alaska for two coupons each way (one each way on United from Seattle-Tacoma to Alaska) on lines that fly to those places; one US Airways coupon is good for a round trip within Florida. Once you buy, coupons are good for a year; once exchanged, the ticket you get is good for up to another year. Unused coupons are nonrefundable.

Travel requires a 14-day advance purchase but no minimum or maximum stay. In fact, you can fly just one-way and return by car, Amtrak, or whatever. You can travel any day of the week, and you can also fly standby. Coupon travel earns frequent flyer credit. Seats are limited and subject to some blackout dates. There is no limit on the number of times you can visit any one city.

Although the basics are similar, three differences may be critical to some travelers:

— Only TWA sells books for companions of any age. They cost $100 more than the senior book; as usual, the companion must accompany the qualifying senior on all flights.

— Up to two grandchildren, age 2 to 11, can use US Airways coupons from a qualifying senior's book.

— Only United and US Airways allow you to upgrade senior coupon tickets with frequent flyer mileage.

America West and Midway also sell senior coupons at prices a bit lower than those of the giant lines. Rules are a bit different, too.

These rules change as the airlines make or lose money. You need to check with your travel agent to sort out the rules. Again, your travel agent can tell you which airline is best suited for you.

Are senior coupons a good deal? I believe they are, when used to maximum advantage. The fixed coupon rate of $350 or less, round trip, is usually lower than the lowest any-age fare for long trips on a giant line. Senior coupons are the only way I know to get a good price on a giant airline without staying over a Saturday night, and senior coupon standby is the only way to get a good price without booking 14 to 21 days in advance.

But don't expect that senior coupons always get you the lowest fare. Those short-term last-minute weekend fares are often lower than the senior coupon price, even for long trips. During frequent fare wars, you can often find any-age fares that are less expensive than senior coupons for short and medium-haul trips. Four-coupon round trips to Hawaii are very bad buys for West Coast travelers. And, of course, with senior coupons you have to pay up front for two round trips, not a good idea of you're planning just one trip.