The sandwich is a food item typically consisting of two slices of bread between which are laid one or more layers of meat, vegetable, cheese, or other fillings, together with optional or traditionally provided condiments, sauces, and other accompaniments. The bread is often either lightly buttered, covered in a flavoured oil when baked, or oil is added into the sandwich to enhance flavour.
Sandwiches are commonly carried to work or school in lunchboxes or brown paper bags (sandwich bags) to be eaten as the midday meal, taken on picnics, hiking trips, or other outings. They are also served in many restaurants as entrées, and are sometimes eaten at home, either as a quick meal or as part of a larger meal. As part of a full meal sandwiches are traditionally accompanied with such side dishes as a serving of soup (soup-and-sandwich), a salad (salad-and-sandwich), or potato chips and a pickle or coleslaw.
Variations
he term "sandwich" has been expanded—especially in the United States—to include items made with other "breads" such as tortilla, rolls and focaccia. Thus hamburgers and "subs", for example, are called "sandwiches" in the United States, although not in the midwest, south or western states or most other English-speaking countries (since they are not made with slices of bread from a loaf).
The nearest traditional Scandinavian equivalent is generally known elsewhere as an "open" or "open-face" sandwich, i.e. a single slice of bread with meat, fish, cheese, etc. as a topping, although the sandwich with two slices of bread has become more commonplace in recent times. This open-face variation is also prevalent in Russia, where it is known as a buterbrod (бутерброд, from the German word for "buttered bread").
In India, sandwiches are often vegetarian, the most common type being the vegetable sandwich.
In the UK, particularly in the north of England they are known, informally, as 'butties' or 'sarnies'. This is particularly the case with sandwiches including freshly-cooked bacon and butter, though other forms of 'butty' use other ingredients and mayonnaise. A sandwich filled with chips (US: french fries) is known as a 'chip buttie' (also butty). In French countries you might see this referred to as un Belge: a Belgian (sandwich). Another informal English name is 'sarnie'. In Scotland, sandwiches are called 'pieces'. One Australian slang term for sandwich is 'sanger'. In South Africa sandwiches are sometimes called 'sarmies'.
Origin
The sandwich was named after John Montagu, 4th Earl of Sandwich, an 18th-century English aristocrat, although it is unlikely to have been invented by him. It is said that Lord Sandwich was fond of this form of food because it allowed him to continue gambling while eating. The name of the earldom comes from that of the English village of Sandwich in Kent—from the Old English Sandwic, meaning "sand place".
Nowadays some types of sandwich are too unwieldy to be held in one hand, thus defeating Montagu's original purpose, and must be eaten with a knife and fork, or at least with both hands. In some countries it is considered proper always to use cutlery to eat sandwiches.
Industry
According to the Washington Post, the USA sandwich business was worth $105 billion US in sales in 2003, with a 6% annual growth.
British Sandwich Week, organised by the British Sandwich Association which represents the sandwich industry in the United Kingdom , occurs on the week beginning on the second Sunday of May. In many economically deprived regions of England and Wales the sandwich is the only school provided lunch for those students who cannot afford to buy their own lunch.
Mathematics 
The sandwich theorem, frequently used in calculus and real analysis, states that if a real-valued function (the filling) lies everywhere between two other real-valued functions (the bread) which both converge to the same limit, then the "middle function" also converges to that limit.
The ham sandwich theorem can be used to prove mathematically that a single cut can divide two pieces of bread and the filling each exactly in half.
Examples
Sandwiches vary greatly both in their style—how they are put together—and their fillings. Not every style can be used with every filling.
