This specifies nodes to be used as the center of the layout and the root of the generated spanning tree. As a graph attribute, this gives the name of the node. As a node attribute, it specifies that the node should be used as a central node. In twopi, this will actually be the central node. In circo, the block containing the node will be central in the drawing of its connected component. If not defined, twopi will pick a most central node, and circo will pick a random node.

If the root attribute is defined as the empty string, twopi will reset it to name of the node picked as the root node.

For twopi, it is possible to have multiple roots, presumably one for each component. If more than one node in a component is marked as the root, twopi will pick one.