At noon, the line for the just-opened Potbelly Sandwich Works at 17th and L streets NW in downtown Washington snakes out the door and onto the sidewalk. Two blocks down, at the Potbelly at 19th and L, it's the same story. There's also a long line for sandwiches at the Corner Bakery on 14th Street, at the Subway on 15th, and at the Quiznos Sub on New York Avenue.

The sandwich business is booming. The National Restaurant Association calls it "one of the brightest sectors in the industry." Data from Technomic, a Chicago research firm that specializes in the restaurant industry, shows sandwich sales growing at twice the rate of the restaurant industry as a whole. Yes, there was a slight dip in sales earlier this year as the low-carb craze hit its peak, but no more. At noon, the goal is something quick, inexpensive and healthy, that doesn't require a knife and fork. For the office lunch break, sandwiches rule.

And the king of sandwiches is turkey. Ham, roast beef, Italian -- they all have their fans, but many of the most successful sandwich chains say that turkey is consistently their top seller.

"If you want it to sell, turkey is key," says Scott Davis, senior vice-president and chief concept officer with St. Louis-based Panera Bread, which racked up nearly $1 billion in sales last year at its nearly 700 bakery-cafes, thanks primarily to its sandwiches.

Turkey's popularity makes sense. It's mild-tasting, a neutral platform for all the toppings and condiments that many chains are offering customers who love to customize. Want cheese, onions and olives on that sandwich? Or how about lettuce, tomato and hot peppers? Or maybe you want it with everything? Turkey takes it all.

"It's fail-safe," says Todd Weisenstein, general manager of the Potbelly Sandwich Works at 17th and L streets NW. "When people don't know what they want, they're always safe with turkey."

But what exactly is the quintessential turkey sandwich? Some chains serve a version like Mom used to make -- sliced bread with lettuce, tomato and mayonnaise. Period.

Other chains go the sub route -- a long roll (white or wheat) packed with shredded lettuce, tomato, cheese, hot peppers, pickles, mayonnaise, mustard, Italian dressing, avocado, salt and pepper, and, oh yeah, turkey.

And then there are the crisp, toasted rolls vs. the soft, cool rolls. And the white bread slices vs. the whole grain. And did you want chips with that? Or maybe baby carrots?

In search of a great turkey sandwich, we decided to try six from the most popular chains (see results of our test, at left). What we discovered is that most turkey sandwiches use deli turkey (those thin slices of bland, vaguely turkey-fied meat), but that enough toppings and decent bread can make up for a lot.