A park system, also known as an open space system, is a network of open spaces which are connected by public walkways, bridleways or cycleways. In modern landscape practice, the park system concept is being overtaken by the idea of planning greenways which run through urban and rural areas.
One of the earliest park systems, in London, came into existence more or less by chance. As London expanded round former royal parks in the nineteenth century, St. James's Park, Green Park and Hyde Park became part of the urban area. This arrangement was admired in France and adopted for the nineteenth century re-planning of Paris by Baron Haussmann. It was also admired by Frederick Law Olmsted and used to create the famous Emerald Necklace in Boston. In 1927, the Maryland-National Capital Park and Planning Commission was formed to plan and acquire parklands along stream valley corridors in the then-rural northern and eastern suburbs of Washington, D.C. Over 33,000 acres (130 km²) are now protected in the Montgomery County, Maryland, portion and provide welcome green space in this urbanized region. A major proposal for a park system was included in Patrick Abercrombie's 1943-4 County of London Plan.