A hill chain, sometimes also hill ridge, is an elongated line of hills that usually includes a succession of more or less prominent hilltops, domed summits or kuppen, hill ridges and saddles and which, together with its associated lateral ridges and branches, may form a complex topographic structure. It may occur within a hill range, within an area of low rolling hill country or on a plain. It may link two or more otherwise distinct hill ranges. The transition from a hill chain to a mountain chain is blurred and depends on regional definitions of a hill or mountain. For example, in the UK and Ireland a mountain must officially be 600 metres or higher, whereas in North America mountains are often (unofficially) taken as being 1,000 feet high or more.