From Wikipedia
A club sandwich, also called a clubhouse sandwich, is a sandwich of occasionally toasted bread, sliced cooked poultry, fried bacon, lettuce,tomato, and mayonnaise. It is often cut into quarters or halves and held together by hors d'œuvre sticks. Modern versions frequently have two layers which are separated by an additional slice of bread, though this is not a required feature of the sandwich.
Other sources, however, find the origin of the club sandwich to be up for debate. Another theory is that the club sandwich was invented in an exclusive Saratoga Springs, New York gambling club in the late 19th century.
The sandwich is known to have appeared on U.S. restaurant menus as far back as 1899. The earliest reference to the sandwich in published fiction is from Conversations of a Chorus Girl, a 1903 book by Ray Cardell. Historically, club sandwiches featured slices of chicken, but with time, turkey has become increasingly common. As with a BLT, toasted white bread is standard, along with iceberg lettuce, bacon, and tomatoes. The sandwich is traditionally dressed with mayonnaise. Variations, however, on the traditional club sandwich abound. Some vary the protein, for example, a "breakfast club" that includes eggs or a "roast beef club." Others include ham (instead of, or in addition to bacon) and/orcheese slices. Vegetarian club sandwiches often include hummus, avocado or spinach, as well as substitute the real bacon with a vegetarian alternative. Mustard and sometimes honey mustard are common condiments. Upscale variations include, for example, the oyster club, the salmon club, and Dungeness crab melt.