For type "LineString", the "coordinates" member must be an array of two or more positions. A LinearRing is closed LineString with 4 or more positions. The first and last positions are equivalent (they represent equivalent points). Though a LinearRing is not explicitly represented as a GeoJSON geometry type, it is referred to in the Polygon geometry type definition.
See also
Other geo types:
Feature,
FeatureCollection,
GeometryCollection,
MultiLineString,
MultiPoint,
MultiPolygon,
Point,
Polygon,
geojson-types