A base graph is an underlying graph relative to which another graph construction is defined. In a graph cover 
, the graph 
is the base graph, while 
is the covering graph.
Vertices and edges of 
are said to lie over their images in the base graph.
In voltage graph theory, a voltage graph is a base graph with voltages; its derived graph is a lift over that graph (roughly, a graph lying above the base graph whose vertices and edges project down to the base graph, with incident edges matching locally).
