A linear settlement is a (normally small to medium-sized) settlement or group of buildings that is formed in a long line. Many of these settlements are formed along a transport route, such as a road, river, or canal. Others form due to physical restrictions, such as coastlines, mountains, hills or valleys. Linear settlements may have no obvious centre. A linear settlement is in contrast with ribbon development, which is the outward spread of an existing town along a main street, and with a nucleated settlement, which is a group of buildings clustered around a central point.