A signed graph is a graph where relations are of two opposing types, typically positive or negative, is called signed graph. If the sign function is \( \sigma \), the we write \( (G, \sigma) \). More formally we have a set of vertices and two multiset of binary relations; the positive ones and the negative one. The elements of these relations are the edges. A edge is a loop if it connects a vertex to itself.
A digon is a pair of identical relations, one positive one negative.
When \( \sigma \) is of little importance, we may write \( \widehat{G} \).
In drawing, negative edges are in red and positive edges are in blue.