A tomato sauce is any of a very large number of sauces made primarily from tomatoes, usually to be served as part of a dish (rather than as a condiment). Tomato sauces are common for meat and vegetables, but they are perhaps best known as sauces for pasta dishes.
Tomatoes have a rich flavour, low liquid content, very soft flesh which breaks down easily, and the right composition to thicken into a sauce when they are cooked (without the need of thickeners like roux). All of these qualities make them ideal for simple and appealing sauces. The simplest tomato sauces consist just of chopped tomato flesh (with the skins and seeds optionally removed), cooked in a little olive oil and simmered until it loses its raw flavour, and seasoned with salt.
Water (or another, more flavourful liquid such as stock or wine) is often added to keep it from drying out too much. Onion and garlic are almost always sweated or sautéed at the beginning before the tomato is added. Other seasonings typically include basil, oregano, parsley, and possibly some spicy red pepper or black pepper. Ground or chopped meat is also common.
In countries such as Australia and New Zealand, in southern Africa and the United Kingdom the term 'tomato sauce' is used to describe a condiment similar to that known in the USA as 'ketchup'. In Britain, both terms are used for the condiment.
The sauce tomate of classical French cooking, as codified by Auguste Escoffier, consists of salt belly of pork, onions, bay leaves, thyme, tomato purée or fresh tomatoes, roux, garlic, salt, sugar, and pepper. Many times, butter and flour will be listed in the ingredients, but those are only used to make the roux (thickening agent). Roux is made of equal parts by weight of flour and butter. Any extra flour or butter that is called for in the recipe is typically an error. 
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