The technical name for a double sugar is a disaccharide. Disaccharides are composed of two monosaccharides, or simple sugars, that are joined together. The most common simple sugars are fructose and glucose, which can join together to form the disaccharide sucrose, commonly known as table sugar.
Other examples of double sugars include lactose and maltose. Lactose is the type of disaccharide found in milk and other dairy products, which consists of one molecule of glucose joined to one molecule of galactose. Galactose is only found in milk, which is why lactose can only occur in dairy products.
Maltose is a more rare form of disaccharide that is typically found in some vegetables and grains, such as barley. Due to its presence in barley, maltose is the primary disaccharide found in beer, which occurs when two glucose molecules join together as certain starches begin to break down.