Wikipedia says:
There are, however, additional named items not on the menu, but available at every In-N-Out. These variations reside on the chain's "secret menu," though the menu is accessible on the company's web site. These variations include 3x3 (which has three patties and three slices of cheese), 4x4 (four patties and four slices of cheese), Neapolitan shakes, grilled cheese sandwich (comes with everything that the burgers come with except meat, plus two slices of melted cheese), veggie burgers (comes with everything that the burgers come with; is not an actual veggie patty, and does not come with cheese), and Animal Style, a house specialty that the company has trademarked because of its association with the chain. An Animal Style fry comes with two slices of melted cheese, spread, and grilled onions on top; Animal style burgers have mustard fried into the meat patties as they cook, and in addition to the lettuce and tomato it also includes pickles, grilled onions and extra spread.[30]
Until recently, it was a trademark of In-N-Out to accommodate burger orders of any size by adding patties at an additional cost. A particularly famous incident involving a 100x100 (100 patties, 100 slices of cheese) occurred in 2004.[31] Once word got out of the incredibly large sandwich, In-N-Out management disallowed anything larger than a 4x4.[32] However, one can order what is called a "Flying Dutchman" which consists of two meat patties and two slices of cheese by itself (no bun, condiments, or veggies).
