The new seats will be available to book starting in May.

Top-tier frequent fliers and customers that buy full-fare economy tickets will be able to sit in the roomier seats with no extra charge. Lower-level frequent fliers will get discounts on the seats.

The seats, located in the first few rows of coach, are similar to those offered by Delta Air Lines Inc. partner Air France-KLM. Those airlines’ seats offer as much as 6 inches additional legroom. Those who pay extra for Delta’s new seats will also get to board early and get free alcoholic drinks during the flight.

Several other airlines offer premium economy seats for a fee, including United Airlines and Virgin Atlantic.

Premium economy seats are not only designed for coach passengers looking for more comfort. They’re also a move by airlines to attract business travelers, some of whom have opted for coach class to save money coming out of the recession.

Delta says that some business-class travelers are coming back, and it’s adding more special seats for them, too. The airline also said Monday it plans to install 34 flat-bed Business Elite seats along the aisles of 32 A330 aircraft within the next two years. The latest move means that Delta will have Business class seats that convert to beds on all of its big international planes, more than 150 of them, by 2013.